This can be anything from the grammar, certain words and phrases, how you’re supposed to speak, read and write etc.
For example French they teach you that the ne + pas is necessary “Je ne sais _pas_” but in reality when we French people speak (at least when I speak) we don’t bother doing that and simply say things like “Je sais pas” or even more simply “ché pas”. To the point where there’s whole videos dedicated to explaining this.
I have been corrected before by teachers for failing to say “Je ne sais pas” instead of “Je sais pas” to the point where I try to now make an active effort to remember to say the ne in spoken language.
I guess for British English there’s also the famous phrase innit meaning “isn’t it”.
Interesting to see how what gets taught in schools/books/in general is different to what actually applies in practice.