The seat mat (also known as the seat occupancy mat sensor or child seat recognition sensor) is a sensor which detects if someone is sitting on the seat. It also detects if the child seat is mounted on the passenger seat. The seat mat sensor is usually mounted in the front passenger seat but some car manufacturers also mount it in the rear seats. The sensor is hidden inside the seat cushion.
When the passenger sits on the seat and his weight is detected the car activates the passenger airbag. In case of an accident the airbag will deploy. When the seat mat does not detect any person (weight) the passenger airbag is not activated and it will not deploy in case of an accident. The seat mat sensor acts differently when the child seat is mounted on the seat. When the child seat is detected, the seat mat does not activate the passenger airbag to avoid deploying the airbag which my hurt a child in the child seat.
When the seat mat is damaged the instrument cluster will show information about a problem with the SRS system. Also the airbag warning light will be turned on. In that state the airbag is turned off and will not deploy in case of an accident. This situation requires to replace the passenger seat mat sensor with a new sensor. This is a very expensive solution. The brand new sensor usually costs about 300€ and more (it depends on the model of the car). Plus you need to keep in mind an additional cost for dismantling the seat and inserting the new mat into the cushion. The seat mat emulator is a less expensive option and there is no need to dismantle the seat. The assembly of the most emulators requires disconnecting the seat mat connector and connecting the emulator.