Tennyson's speaker repeats the word "break" three times in two different stanzas of the poem. Repetition is used for emphasis, and Tennyson is trying to emphasize the relentless way waves continually crash on a shore. They never stop. They break, or crash, over and over again.
Tennyson is contrasting this seemingly eternal, timeless breaking of the waves with the passage of a human life. After noting the pleasure of a "fisherman boy" and a "sailor lad," who are too young to realize how fleeting life is, he turns in the final two stanzas to his own fate.
As the waves relentlessly and remorselessly break and break and break, the speaker mentions a beloved person in his life who has died. The ending of this person's life is an echo of the endless breaking of the waves—and perhaps of the endless breaking of the speaker's heart.