A lot of the marks people lose at GCSE are not due to a lack of understanding, but due to not reading the question properly and having poor exam technique (e.g. only describing when the question says explain). I'm sure your teachers will cover this with you in a lot more detail, but if they don't, do look at old exam papers with the mark scheme and try past questions (use other exam boards or the old syllabus if you run out) to practice your technique.
While the videos are good, active revision (flash cards, making notes, mind maps) tends to be better than passive revision for really making things stick.