@MaMaMaBelle A glottal stop is the sound / non-sound in the middle of a word like "waiting" or "butter" if you imagine it pronounced in a cockney accent. It's sort of a catch in your throat that makes a short, sharp pause in place of (in some UK accents) a "t" sound. In some languages it's a functioning part of the language's phonology, so a word pronounced mata would have a different meaning than a word pronounced ma.a (where . represents a glottal stop), in the same way as the t/d distinction changes the meaning of the words 'to' and 'do' in English for example. But in English it doesn't have a meaning-changing function, it's just accent dependent. So some people are aware of it; others don't really notice it.
I'm a linguist by training / profession. There are so many regional accents in the UK that you can't really argue for "correct" or "incorrect" pronunciation based on every usage of a sound - e.g. it's not factual to say "glottal stops are a feature of incorrect pronunciation", because they're not; they're a feature of particular regional accents (and are "correct" in those accents, in the sense that they're subject to the same consistent phonological rules as any other sound that forms part of that accent). It is of course possible to mispronouce individual words e.g. people who pronounce ibuprofen eye-broo-fen (I'm looking at you, DH). And there are some cases where you could argue either way, e.g. ahks for 'ask' - in some accents this word is consistently pronounced this way, but in others it would be considered incorrect. So the concept is a bit fluid.
The reason there's a perception that RP pronunciation is "correct" is just due to social conditioning. For so long, that was the accent chosen to formally represent spoken English by royalty, government spokespeople, newsreaders etc that a stigma was attached to other regional accents. But regional variation almost always has a historical basis and one accent isn't "more correct" than another from a linguistic point of view; it might just be more socially accepted from a social conditioning point of view.