Literally, when the speaker tells the sea to "Break, break, break / At the foot of thy crags," he is ordering the sea to go ahead and crash into the rocks at the base of the land, perhaps the land on which he stands. The "thy" to which he speaks is the sea, and this is a technique called apostrophe, when the speaker addresses something that cannot respond as though it could. However, this "break[ing]"—including the repetition of the word "break" three times—is juxtaposed with the next, intangible, and much softer idea: "the tender grace of a day that is dead / [Which] Will never come back to me." The speaker is mourning someone who has, apparently, died, someone who imparted a "tender grace" to his days. Though, now, he feels that his life will lack this tenderness and, instead, be characterized by "break[s]" which are painful and violent. Thus, when the speaker references the "foot of thy crags," he isn't just referring to the literal rocks, but also to the bleak harshness of his life after he has lost his loved one.