Rhyming iambic pentameter lines are the most common form and meter in Frost's early poetry, or at least in the poems from his first two collections, A Boy's Will and North of Boston. Iambic pentameter lines have five "iambs" – rhythmic units of a short, unaccented syllable followed by a longer, accented syllable. Iambic pentameter sounds like da-DUM da-DUM da-DUM da-DUM da-DUM. Here's an example from lines 21-22:
My in|-step arch | not on|-ly keeps | the ache,
It keeps | the pres|-sure of | a lad|-der round
But the rhymes in "After Apple-Picking" do not follow a set pattern. Sometimes a rhyme occurs at every other line, sometimes at every fourth line, and sometimes in consecutive lines. Lines 28 and 29, for example, take the reader by surprise with an abrupt rhyming couplet ("overtired"/"desired") that stands out against the more widely spaced rhymes elsewhere. Such irregularly rhyming verse was used frequently in dramas of the English Renaissance, and it is fitting that the form would be used extensively throughout North of Boston, a collection that contains several longer, dramatic poems.
But we don't think that Frost meant to make any fancy allusions to poetic history – he was a practical poet, and he probably just wrote in the form that he thought sounded best and which came easiest to him. He was certainly not an innovative rule-breaker like his near-contemporary T.S. Eliot. But not everybody needs to be an innovator, and Frost was no copycat. He is an absolute master of this form, and he's also enough of a modernist to know that he can break the rules without hurting the poem. Some of the lines are considerably shorter than the standard ten syllables, like line 32, "For all." Finally, the poem is not divided into stanzas. It is a single, unbroken narrative, similar to other well-known Frost poems like "Mending Wall."