Break, break, break,On thy cold gray stones, O Sea! (1-2)
The sound of the sea waves "breaking" is repetitive and meaningless, setting up the theme of the speaker's worry that his own crying and grief is equally repetitive and meaningless.
And I would that my tongue could utterThe thoughts that arise in me. (3-4)
The speaker claims that he is unable to express his own thoughts, which seems odd, given that he goes on to finish one of the more famous poems in the English language. He phrases his inability rather passively: he doesn't say, "I can't utter what I'm thinking," but rather, says that his "tongue" can't "utter" it, as though his tongue were somehow separate from the rest of him.
O, well for the fisherman's boy,That he shouts with his sister at play!(5-6)
Here's that syllable "O" again – it's a meaningless sound, like the sound of the waves or the happy "shouts" of the "fisherman's boy." The fisherman's boy is not shouting words – at least, the words are not repeated by the speaker – he's just making noise, but without expressing anything. The speaker is worried that that's exactly what he's doing in this poem.