Some cloth diaper liners are very thin and disposable. Many cloth diaper manufacturers also sell rolls of these diaper liners, too. Disposable liners are made to either be thrown in the trash or flushed down the toilet. Some may also be compostable.
Other reusable diaper liners are made of thin fleece or another type of fabric that can sit atop the cloth diaper to protect from diaper creams. It's often easier to strip diaper cream residue out of a small load of reusable liners than to strip a whole load of cloth diapers.