Crickets use a type of ear drum on the front of their legs to feel the vibration from sounds. They cannot hear true sounds but can feel the vibration from the sounds that are being made, which is especially helpful for female crickets looking for a mate.
Males are the only crickets able to make sounds with their legs and do so for different purposes. Male crickets have chirping songs that will help them find females, sounds that allow them to warn other males to stay away, sounds that warn all of the other crickets of danger and songs that let them communicate with the female while mating.
Because the male crickets have so many songs for so many different purposes, other crickets must be very tuned in to what they are singing about. The ear drums that are on a cricket's legs are sensitive enough that they can tell what type of chirping song another cricket is making, even if humans can't tell the difference between the chirps. It is important for other crickets to know what is being sung because it can keep them away from a female that is already being mated; it can also save their lives if another cricket is warning of danger in the area.