A brayer is a rubber roller. It has one job, which is to lay ink down evenly, and it is almost impossible to do well on the first attempt.
Too much ink and the lino fills. Too little and the print comes up patchy. The roller has to travel in more than one direction, and the ink has to sound right on the slab before it goes anywhere near the block.
That sound is the whole lesson. It cannot be described usefully in advance. It has to be heard, in a room, standing next to someone who already knows it.