The explanation below is offered from several online sources, which seem to be attributed most often to Rhina P. Espaillat:
…the “ovillejo,” an old Spanish verse form that means “tight little bundle.” “-ejo” is one of our blessed diminutives, and “ovillo” means “tangled ball of yarn.” The last line is a “redondilla,” a “little round” that collects all three of the short lines. The rhyme scheme is established, but the meter is at the poet’s discretion, although in Spanish the longer lines tend to be octosyllabic (8 syllables).
The ovillejo is an old Spanish form popularized by Miguel de Cervantes (1547-1616). This 10-line poem is comprised of 3 rhyming couplets (or 2-line stanzas) and a quatrain (or 4-line stanza).
The first line of each couplet is 8 syllables long and presents a question to which the second line responds in 3 to 4 syllables–either as an answer or an echo.
The quatrain is also referred to as a redondilla (which is usually a quatrain written in trochaic tetrameter) with an abba rhyme pattern. The final line of the quatrain also combines lines 2, 4, and 6 together.