What is a trap primer for

It is a device designed to squirt water into a p-trap whenever there is a change in system pressure.

How it operates has nothing to do with its intended use, by the way.

Its intended use is to continually add water to a p-trap that doesn't see regular use, such as a basement floor drain or an indirect drain used to collect condensate from an air conditioner.

If calibrated correctly, the trap primer will squirt out a quick burst of water every time it senses a sudden change in pressure.