William Wordsworth's poem "The Solitary Reaper" has a mixed rhyme scheme, with the first and last stanzas following an abcbddee pattern and the middle stanzas following an ababccdd pattern. More importantly, the poem is written in iambic tetrameter. Similar to iambic pentameter, iambic tetrameter is slightly more compact, as it uses only four feet per line, rather than five. A foot is a group of two syllables, one of which is stressed and one of which is not stressed. Each line in Wordsworth's "The Solitary Reaper" contains four of these groupings, thus ensuring that a constant, reliable rhythm runs throughout the verse and holds the whole composition together.
Understanding poetic feet can be difficult at first, so here's a trick to start figuring it out: when looking at a line of poetry, count the number of groupings of two syllables. The ending number will be the total number of feet. Once you get the hang of this, it will be much easier to also identify stressed and unstressed syllables, and also whether something is written in iambic pentameter or not. Try it out with the lines in "The Solitary Reaper," and aim to identify four feet per line.