In general it just means going over more information on the same topics to make sure you fully understand them. Doing practice questions etc.
If you Google revision books or resources for the A levels you do, you'll find plenty of things to use.
My teachers say we need to do extra work at home as the stuff we cover in lesson is only the bare minimum but what does this mean idk what to look for or do...
Depending on the subjects. It could be as simple as reading a book relating to it. For example, I do Classical Civilisation and I had to known four Greek tragedies. I read before the exam in own time due to interst just one more different play, hence I got 98% on it. I thought it was a coincidence as it was the only subject I did extra work for. But it worked and I know to do that this year.
What subjects do you take?