Break, break, break At the foot of thy crags, O Sea! But the tender grace of a day that is dead Will never come back to me.
The speaker repeats the first line again, telling the waves to "break, break, break" again.
But it's repetition with a difference: in the first stanza, he tells the waves to break "on thy cold gray stones," and in the last stanza, he tells the waves to break "at the foot of thy crags."
It's not exactly the same – time has gone by, and even the breaking of the waves has changed slightly. Maybe it's the tide coming in.
The waves have changed slightly, and we see that time is passing, despite the tragedy that the speaker has suffered. Mournfully he says that the happy old days when his friend was alive will never return.