Brutus wants to march to Phillipi. In other words, he want to take to battle to them. Cassius wants to stay put and let the battle come to them.
Cassius believes that if they stay where they are, Anthony and Octavius will need to come to them, thus wearing their troops out before the battle itself. By staying where they are their troops will be fresher for the battle.
Brutus feels that it is better to take their troops and meet Anthony and Octavius at Phillipi since the people in between where they currently are and Phillipi cannot be trusted to be loyal to them. It would be better for them to show these people a show of force to keep them in line.
Brutus wins the arguement and they march to Phillipi where they are defeated.