As with any theory, this one is good to know at GCSE and essential at A-Level, where you should also refer to it by name.
Life is full of opposites. Some opposites exist at opposite ends of a large scale. For example, 'light and dark' are two ends of a scale, but there are degrees of light or darkness that exist in between. It can be quite dark, a little bit dark, quite light, very light and so on. The same is true of opposites like 'hot and cold', 'rich and poor', 'old and young'... the list goes on! Because there are lots of options between the two opposite ends of the scale, we call these non-binary opposites. The word binary comes from the Latin bini, meaning 'two things together.' If something is non-binary, then there are more than two options, responses or possible states of existing.
Binary opposites on the other hand have no in between options, and are much rarer. 'On or off' is one of the best examples. 'Yes or no', 'Present or absent' or 'dead or alive' are all good examples.
That said, we are increasingly as a society taking even ideas that were thought of as binary and re-evaluating them as non-binary instead. Our changing thoughts on gender as a society are perhaps the best example of this. If you are spiritual, you might not see 'life and death' in such binary terms.
You might even have light switches at home that can be on, off or dimmed!
Binary oppositions are quite rare in the real world because life is far more complicated and nuanced, as is our way of thinking and talking about it - and as a result, they're becoming rarer.
Claude Lévi-Strauss' Structural Theory says that the world of media, on the other hand, is driven entirely by binary opposites. In media narratives, there is no room for the complexity or nuance of real life, lest the message be lost on the audience. Things need to be clear, simple and with as few options as possible.
You'd think that would be just one option, but here is the important thing about binary opposites: one has absolutely no meaning without the other. If you had never experienced evil in the world, you wouldn't know what good felt like. If you lived in a constant winter, you would have no concept of 'cold' because you would never have experienced being hot. Emotions in particular have little meaning without experiencing their binary opposite. Sadness would have no significance if you had no recollection of feeling happy. In media narratives, every thought, feeling or idea has its natural opposite with little (and often no) in-between. Quite often, media pieces will present a binary opposition, with one option represented as the one we should desire and aspire to. The other (in every sense of the word) is presented as undesirable and in need of being avoided or lessened. Media must be careful, however, not to allow one option to disappear entirely, otherwise the preferred option becomes meaningless! The undesirable option always needs to be in the background as a sort of threat or motivator.
Good vs evil is an excellent example from fictional media narratives. Characters are one or the other, with (rarely) any in-between as this makes it much easier for the audience to follow the characters and plot. Indeed, there are links with Steve Neal's Genre Theory here in that good vs evil is a repeated idea of many film genres. Each will always try to destroy the other, and this is what drives the plot forward, with good being the side that we are supposed to favour. Success and defeat is another good example, with films and TV shows less often showing ambiguous outcomes - we usually expect the narrative to end with a heroic victory and a villainous defeat. We also know that the villain is never fully defeated, otherwise what is the point in the good guys turning up for work in the next episode or film?
In media forms like advertisements, we quite often see binary opposites like happy and unhappy, has not and has, want and fulfilment, desirable and undesirable, fashionable and unfashionable. Adverts particularly enjoy presenting binary oppositions to do with time, especially now and then, with the past often presented in a more favourable light. They often employ the binary opposition of us and them/everyone else. The advert opposite is a great example of these latter two. Adverts will place us with the negative opposite (or threaten us with it) and promise, through their product or service, to put or maintain us on the better side instead. Here again, the wider system is careful not to totally remove want, not having or being in an undesirable situation so that it can keep selling us products and services. It's also important that advertising in particular has a tricky relationship with choice. Certainly, we have a choice between products (e.g. Pepsi, Coca-Cola), but though the advert seemingly presents us with a choice (buy or miss out) it's actually an illusion. While we may feel like we have made a choice to purchase the product, quite often the advert has made that decision for us because the alternative is supposedly unbearable!
In-summary: The real world is quite complex, and many things exist on a non-binary scale with lots of options. Binary opposites, with just two options, are rarer. Science, technology and social attitudes are driving a change whereby things we thought of as binary are increasingly being explored as non-binary instead.
In Media, however, it is much simpler. Lévi-Strauss says that most media texts are driven by binary oppositions, which are simple for audiences to follow and run the least risk of making the message unclear. Media will present one option as best avoided and the other as one that we should support or strive for.