No surprises here. The poem is called "Metrical Feet" because the first half is all about different types of poetic meter. Poetry is written in a meter, which is a fancy way of saying "beat." Lines of poetry are divided into little groups called "feet" which usually contain two or three syllables. The organization of each foot is what gives it a cool name like iamb, trochee, spondee, dactyl, amphimacer, amphibrach, anapest… you get the point, we'll spare you the mouthful.
Coleridge explains each of these types of "metrical feet" in the first stanza of the poem, and he does so in such a way that, theoretically, should be fairly easy to memorize (it rhymes, uses lots of repetition, and all that jazz.) Although Coleridge wrote the poem in order to explain the meters of ancient Greek to his young son, it is clear that the same rules apply to English (only we speak of unstressed and stressed, rather than short and long).
The other part of the title – a "lesson for a boy" – tells us about the poem's purpose. Coleridge imagined it as a little teaching tool for his young son (who wasn't even seven when Coleridge gave him the poem), and this explains why he repeats lots of sound and uses neat little rhymes like "ridge" and "Coleridge." They are the kinds of things that help us learn and remember things.
So if the poem is about the different kinds of "metrical feet," it is also, at least in part, a "lesson" about how to become a poet. For the speaker of the poem, this involves a good knowledge of meter, a "tender warmth" in one's heart, and a deep love of nature. Good advice, Coleridge – we'll get right on that.