"Metrical Feet" is a poem for a "boy," and Coleridge wrote it in order to help his son learn about different types of metrical feet. The poem's clever rhymes and puns function as teaching tools to help a student remember things about poetry in the same way that rhymes like "i before e except after c" do. In addition, the poem also includes a small comment in the second stanza emphasizing the importance of its lessons and of learning more generally. If we had to describe this poem in one sentence we'd say: it's a teaching poem, and it's meant to educate its reader.
The poem suggests that a person's success later in life depends on how well they remember things they have learned as children.
The poem's use of rhymes, puns, and other playful forms of language show that these are more effective teaching tools than memorization or testing.