This is a trickier question than you might think. The poem you're reading, "How do I love thee? Let me count the ways," doesn't actually have a title. The sonnet sequence that it's a part of is titled Sonnets from the Portuguese, even though none of them were in Portuguese to begin with.
Confused? Don't be. Elizabeth Barrett Browning wrote 44 sonnets for her husband, Robert, in the 1840s. In 1850, she published them as a series, and she titled it Sonnets from the Portuguese. This made readers think that Barrett Browning had simply translated sonnets that she originally read in Portuguese, whereas in reality she had written them herself in English. "My little Portuguese" was an affectionate pet name that Robert used for Elizabeth, so by calling them sonnets "from the Portuguese," she was essentially saying "to Robert from Elizabeth" in a secret way that most readers couldn't decode.
The individual sonnets don't actually have titles, and so we do what we must when a poem is untitled: we call it by its first line. So this sonnet, number 43 in the sequence Sonnets from the Portuguese, is known by its first line, "How do I love thee? Let me count the ways" or "How do I love thee?" for short. And that pretty much says it all: the poem is a list of ways that the speaker loves her beloved. It's interesting to notice the way that the poet balances this list structure with a traditional sonnet structure, which requires a surprising turn at the end.