Pre-felt is, as it sounds, a piece of felt that hasn’t been fully felted but remains soft and can usually be cut up and put into further felting. Occasionally, if you’ve felted the wool a little too far it can be difficult to get the pre-felt to stick without stitching or needle felting it on. But if your pre-felt looks like this
You can see most of the fibres have already locked down and a skin has been formed. Using either a hand carder or dog brush just brush the fibres on the back of the pre-felt until they’re all fluffy.
Your pre-felt will then adhere to further felting without the need for stitching.