When Julius Caesar and Brutus were boys, they had thier horoscopes read. The astrologer would then say what was ahead for them in thier futures. When Caesar said this, he meant that what was wrong with them and thier lives was not because thier stars or horescopes had said so, but because they had made their lives bad, and corrupted themselves.

Cassius says this, not Caesar, and means that they control their destiny not fate. Not the stars, but their future lies within themselves.