These lines are found near the end of act 5, scene 1, as Brutus and Cassius prepare for battle. In this conversation, they are discussing the many unknows which lie ahead of them and trying to imagine how it all might play out.
Brutus's thoughts return to Cato, whom he judges harshly for earlier committing suicide. He finds such actions "vile" and "cowardly," certainly not the way for a great warrior to meet his end. Brutus sees Cato as weak, a man of fear who couldn't face "what might fall." Cato therefore avoided any possible negative outcome by taking matters into his own hands, cowardly avoiding the future suffering that might come to fruition later.
In contrast, Brutus believes in staying the course. He has committed to this plan and submits himself with "patience" to allowing "some high powers" to determine his fate. Thus, the same principle by which Brutus judges and condemns Cato pushes Brutus to be a different kind of man—one who faces the possibility of an unexpected or negative outcome directly. Brutus accepts that things may not go well for him personally, yet he refuses to live in such fear of "what might fall" that taking his own life, as Cato did, seems appealing.