A nappy liner is designed to lay between bubs skin and the nappy.
Usually about 30cm x 12cm for daytime, and a contoured nappy shape for night time.
Polar fleece is popular .. it wicks moisture to the core of the nappy leaving bub feeling dry and helping prevent nappy rash. As well as protecting your nappies, it is easier to clean. Even messy poo slides off fleece easily (I like to hold the liner over the toilet and flush).
Raw silk liners have natural healing properties and have been known to heal nappy rash and sooth skin. It is antifungal and antibacterial for up to 200 washes (many say more!). They have wicking qualities to a lesser degree than fleece.
Flushable nappy liners are great for outings, or when bub is sick with the runs. The whole liner can be flushed down any loo system or pit, which means no more carrying poo around in your nappy bag! My prefered brand is made from 98% unbleached paper pulp and 2% viscose.