Pressed wool blanket, thickness 4 mm
When a print is being made the blanket pushes the paper fibres into the grooves in the plate. These grooves are filled with ink. When the paper is pushed into the grooves the paper will absorb the ink.
Don't use more felt than necessary!
Blankets are made of wool. A glue keeps the wool fibres together. The glue makes the wool slightly stiff. This is the reason that some printmakers, in specific cases, prefer woven blanket. (very fine details, for example)