Liners sit on top of the diaper, in between it and baby's bum to form a barrier.
Reusable liners (e.g. fleece) keep baby feeling dry, and protect the diaper from creams and stains.
Disposable liners make poop cleanup super easy - just flush them along with the poop. As with any flushable product, use caution with septic systems or sensitive plumbing as they can create clogs.