Made from ... wool tops
Felt is a non-constructed textile created from wool or other animal fibres. Fibres making up the textile are matted together into a non-constructed fabric. Unlike woven or knitted cloth, felt when cut won't fray. Whilst it can be made from raw wool in it's natural state, hand made felt is frequently made using "wool tops".
In the UK "Wool tops" usually refers to the finest wool with the longest fibres. It's a semi-processed product of raw wool. Combing and carding removes dirt and short hairs plus aligns the fibres. What's left - the high quality long fibres - are the wool tops. Worked well these long fine fibres produce a top quality felt.
Traditional felt is made by adding hot soapy water to wool fibres layered in alternate directions and then applying lots of friction. This method is known as "wet felting". Like human hair, wool fibres have scales on their outer surface. The hot soapy water used to water down the wool fibres opens these scales. Agitating/rubbing the fibres together causes the opened scales to interlock. In this way the cloth shrinks as the fibres it is formed from compact, tightening and matting together. The end product is felt.