Tradition in poetry is the body of inherited poems, forms, themes, styles, and techniques that later poets learn from and respond to. A poet may follow tradition, revise it, or push against it, but the older work still shapes what comes after.
Tradition can include everything from fixed forms such as the sonnet to recurring subjects, images, and ideas passed along through a literary culture.
Example
The new poet borrowed the sonnet form from tradition, but turned the final turn toward city buses instead of love:
Fourteen lines, a strict and echoed rhyme,
then one sharp shift: the morning traffic noise
becomes the pulse that measures human time.