Barthes' Semiotics is just about the first theory we teach you in Media Studies. You need to know it and revise it whether studying GCSE or A-Level, and should refer to it by name if using it at A-Level. The theory says that media texts convey their meanings not only in what we immediately see or hear but what it makes us think as a result. In English, you might know this as the signifier and what is signified. In Media, we talk about the denotation and the connotation.
Let's look at this media text. An image. You probably haven't even noticed it, but in a fraction of a second, your brain has fully processed the message, first in what it literally sees and what it means. You're probably feeling some sort of emotional response as well. First, your eyes took the image in. What did you actually and literally see? A skull set atop a pair of bones, one crossed over the other. This was the denotation. That, however, is not the message we're supposed to receive and not the message that you received in the split second after looking at this image. Your brain processed this information and came to a further conclusion. This image has a deeper significance or connotation. We know that this symbol means death, and that whatever it is marked with is extremely hazardous to our health and well being. It's likely that you thought about your own death or demise the moment you saw this image. You might also have thought about a sea voyage (more on that in a second...) This particular symbol, like a number of others, has undergone a special process called naturalisation. Some symbols become 'just known' for meaning certain things and their meaning becomes very fixed and rarely changes. This can happen within a community, an industry... it can even happen within a country or across the world and organisations even exist (like the International Standards Organisation) for this purpose.
It's pretty much globally known, for example, that a skull and crossbones means death, a green light means 'go', the colour red means 'danger', a red cross means medical assistance. In driving, the signs for 'stop' and 'give way' (probably the two most important) are the same in almost every country in the world by design. It's notable, in fact, that some of the most globally naturalised symbols are ones to do with preserving health and safety.
Even more interestingly, symbols can change in meaning depending on when and where you see them or rather in what context. A skull and crossbones means a deadly material or substance in 2021, but in the 1660s it would mean you were about to be boarded by pirates In fact, the skull and crossbones is still closely associated with both in society, despite the Golden Age of piracy having ended over 350 years ago.
If nothing else, the outcome of death has remained consistent over the centuries.
This drift and loss in meaning over time, however, can cause issues, especially as we create messages with significances and intended audiences long, long beyond our own lifetimes. What do you mark a container of hazardous waste with in the 2020s that might be unearthed in the 3020s? Will English even be spoken then and will our symbols for danger have kept their meaning across 1,000 years? In summary Media texts have a literal layer - what we actually see or hear. This is the denotation. We say what we denote. Media texts have a signified layer - their actual message that they want us to think about. This is the connotation. We say that we connote.