Cassius makes his arguments as to why Brutus should join the plot to assassinate Caesar in Act One, Scene Two. Cassius realizes that Brutus is respected by the citizens of Rome, and having Brutus participate in the assassination will justify the conspirators' actions to the masses. Cassius attempts to persuade Brutus into participating in Julius Caesar's murder by convincing him that it is in the best interest of Rome. However, Brutus is torn between the love he has for his friend, Julius Caesar, and his duty to the Roman citizens. Cassius proceeds to remind Brutus that Caesar is not a god and believes that he should not be exalted among other men. Cassius then tells Brutus,

Men at some time are masters of their fates. The fault, dear Brutus, is not in our stars but in ourselves, that we are underlings. (1.2.140–143).

Cassius is essentially telling Brutus that he is in control of his own fate, and encourages Brutus to take control of his destiny. Cassius is saying to Brutus that it would be their own fault if they allowed Caesar to rule over them as slaves. Cassius argues that Brutus would be preserving his legacy and saving Rome if he chose to assassinate Julius Caesar. 

These words occur in scene two of the play. The context is when Brutus and Cassius hear the crowd roar. It seems that the people are praising Caesar for something. Brutus states that he fears that the people will try to make Caesar King. This becomes Cassius's entry point into feeling Brutus out to see if he will join the conspiracy to kill Caesar.

Here it the conversation:

BRUTUS What means this shouting? I do fear, the people Choose Caesar for their king. CASSIUS Ay, do you fear it? Then must I think you would not have it so.

Based on this context, the words of Cassius are intended to bring Brutus over to the conspiracy. Cassius wants to remind Brutus that fate is not what governs the affairs of people or even a nation. People have to make decision. In this way people become masters of their own destiny. To be more pointed, Cassius wants Brutus, a honorable and virtuous man, to join the conspiracy against Caesar.