This quotation is from a discussion about military strategy between Brutus and Cassius. In it, Brutus is speaking to advocate attacking Octavian at Philippi. He argues that this would be the ideal time to attack, before Octavian can augment his forces.

The central concept is one known in Greek rhetoric as "kairos," or the ideal time or moment. It is based on the notion that the right words or actions are not enough; they must also be done at the right time.

The notion of a tide is metaphor for such a time. A ship, trying to land, needs to work with a rising tide. Once the tide peaks, it then starts to recede. Brutus is suggesting that this moment is like that of high tide. Once the moment is passed, there will be no equally favorable situation in the future.

This quotation reflects the Stoic philosophy of Brutus which acknowledges the inevitability of laws of nature and sees human success as based on understanding and following those laws. Rather than fighting against the tide, in human or nautical matters, the wise man chooses to work with those forces of necessity.

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.

This is a metaphor. It means literally that there is an occasion in each man's life when he can commit himself to a course of action that will lead to his success; but if he fails to take the opportunity when it presents itself, for the rest of his life he will be a mediocrity or a failure. Brutus uses this very famous metaphor in Act 4, Scene 3 in order to justify his insistence that he and Cassius fight with Antony and Octavius at Philippi. Brutus concludes his metaphor with these words:

On such a full sea are we now afloat,
And we must take the current when it serves,
Or lose our ventures.

The metaphor is famous because it has so often shown itself to be true. People have to make choices in life. They don't have an infinite number of opportunities, and as they grow older their choices become fewer and fewer. Robert Frost touches on this concept in his poem "The Road Not Taken." The speaker of that poem is looking back at a particular point in his life where he might have taken a different "road." He will never know whether or not he made the right choice. In Arthur Miller's play Death of a Salesman, we learn that Willy Loman had an opportunity to go to Alaska with his brother Ben but decided to remain a traveling salesman covering New England. Ben became rich. Willy is an excellent example of a man whose life "is bound in shallows and in misery." 

Unfortunately, many men do not realize that they had a golden opportunity at one time until that opportunity is only a memory of the past.