To quote the main Enotes page for the poem, "The poem consists of eight quatrains, or four-line stanzas. Each stanza is written in pentameter and trimeter verse with an alternating rhyme scheme." This means that the poem has 32 lines in total. The lines are grouped into fours and separated by space breaks. Lines grouped and separated from other groups by space breaks are referred to as stanzas. In this poem, the stanzas are written in pentameter and trimeter, meaning that the line includes five metered feet (syllables). We study the stressed and unstressed syllables of the poem and identify its pattern (e.g., iambic).
According to Enotes, a rhyme scheme:
is the pattern of rhyme between lines of a poem or song. It is usually referred to by using letters to indicate which lines rhyme. In other words, it is the pattern of end rhymes or lines. A rhyme scheme gives the scheme of the rhyme.
We assign letters (ABAB, or ABABCDCD, etc.) in order to explain the rhyme scheme and help us to identify the poem's form, if any. In this case, the rhyme scheme is ABAB CDCD, EFEF, etc.