A ways and means resolution is needed to authorise the creation, extension or increase of taxes or other charges. Before it’s agreed, a ways and means resolution is called a ways and means motion.
Ways and means motions are most commonly put to the House for agreement immediately after second reading, in which case there’s no debate (because there’s already been a chance to talk about the ways and means motion in the second reading debate).
When the main purpose of a bill is to tax or charge the public (as it is with the Finance Bill), the ways and means motions are debated and agreed before the bill is introduced.