The quote in the question occurs during a conversation between Brutus and Cassius about whether or not their troops should march to Philippi to confront the army of Antony and Octavius. Cassius says he thinks marching is a poor idea. It would be better, he argues, for their army to wait and rest and let the other army tire itself doing the work of marching to meet them. Brutus retorts that Antony's army will pick up fresh troops along the way and grow larger, as the people are on his side. He then says, right before the quote above, that
The enemy increaseth every day.
We, at the height, are ready to decline.
Brutus goes on to use the image of a cresting wave getting ready to crash, saying, "There is a tide in the affairs of men."
What he means is that they have to ride the wave of fortune—in other words, seize the opportunity that now presents itself—or they will lose their chance. Another way of putting this is to understand Brutus as saying, "We need to strike now while the iron is hot."
This is true, as we do know we have to seize opportunities when they come to us—but the question is, has Brutus rightly discerned that their fortunes are at their height? Since his side loses, we might think not.