Fixed verse forms are a kind of template or formula that poetry can be composed in. The opposite of Fixed verse is Free verse poetry, which by design has little or no pre-established guidelines.
The various poetic forms, such as meter, rhyme scheme, and stanzas guide and limit a poet's choices when composing poetry. A fixed verse form combines one or more of these limitations into a larger form.
A form usually demands strict adherence to the established guidelines that to some poets may seem stifling, while other poets view the rigid structure as a challenge to be innovative and creative while staying within the guidelines.