Liners are a thin layer of material which are used to create a barrier between baby's bottom and the inside of the diaper. Often liners are a single use, flushable and biodegradable product, but you can also purchase reusable cloth liners. Biodegradable liners are used to help protect the diaper from getting soiled, making for easier cleaning and less staining. They mostly used to protect diapers from newborn poop. You can purchase a roll of 200 for about 7-10 Eur. To use liners you just tear one off the roll and lay it into the diaper.
Reusable liners are a single layer of a stay dry materials, usually fleece or raw silk. They are made to let wetness pass through and help keep babies skin dry. Reusable liners are shaped like a doubler - a long rectangle. They are not made to add any absorbency at all. Fleece liners will cost about 1 Eur a piece and raw silk will be about 2.50 Eur each. You can easily make your own fleece liners by purchasing some fleece at your local fabric store and cutting it into rectangles. Some babies could be sensitive to fleece through, even it's rare. Diaper liners are good for when you don't need any extra absorbency but do need stay dry protection. You probably won't need liners during the day, when you can change your baby regulary short after it pees, or you are just trying to get use her or him to potty. But stay dry liners are really nice at night so babies don't feel wet and sleep more peacefully, and for babies that wake up with redness after a longer time in a wet diaper. They are thin enough to not add any bulk to diaper, which is nice.