What does "there is a tide in the affairs of men" from Julius Caesar mean?

Brutus speaks these words in Act IV, scene ii in order to convince Cius that it is time to begin the battle against Octavius and Antony. He speaks figuratively of a “tide” in the lives of human beings: if one takes advantage of the high tide, one may float out to sea and travel far; if one misses this chance, the “voyage” that one’s life comprises will remain forever confined to the shallows, and one will never experience anything more glorious than the mundane events in this narrow little bay.