To brush up on something is to refresh a skill you have not used for a while. Brush up on your French, brush up on the rules, need to brush up on it.
It assumes the skill was once there, which is what separates it from learning. It is what people say before an exam, an interview or a holiday abroad.
1phrasebrush up onTo refresh a skill you have not used for a while.
“She spent a fortnight trying to brush up on her Spanish.”
brush up on is a phrase.
Also accepted in games: brush up
Practised in Phrasal Verbs