In Act I, Scene 2, what does Cassius mean when he asks to be the mirror into which his friend Brutus can look to see himself?

Cassius is referring to Brutus' inability to see himself in the same way everyone else sees him. Brutus is unable to see or comprehend his influence, or the degree to which he is respected. Cassius is encouraging Brutus look at the way he sees him, in order that Brutus might be able to discover himself in the process.