Poesy, for Bacon, is not defined by verse — that is only 'a character of style.' What matters is the operation of imagination it exercises: a freedom from the laws of matter that lets it 'join that which nature hath severed, and sever that which nature hath joined,' assembling unlawful matches the actual world does not permit.
Bacon's later remarks (not quoted here at length) go on to credit poesy with a genuine cognitive function: because the actual world often fails to reward virtue as it deserves or punish vice as it should, poesy 'submits the shows of things to the desires of the mind' — supplying a more just and satisfying order than history alone provides. It is a strikingly generous verdict from a philosopher whose whole programme is otherwise devoted to constraining the mind by fact.
Drawn from Chapter III of The Advancement of Learning (Book II, section IV), on the threefold division of learning.