The transmitter is the piece of hardware which ensures that your child can go to sleep confidently. When the transmitter is attached to the underwear with the two snaps, the transmitter is able to detect wetness. As soon as this happens, the transmitter sends a message to the receiver to wake your child up.
The battery of the transmitter lasts long enough to achieve dry nights with the bedwetting alarm method. If the battery is empty, you can open the cover and replace the button-cell battery.
If you have purchased a new transmitter, you can easily couple it with your receiver. The app gives you clear instructions on how to do this.