What is Pulse Output?
Dry Contact Reed Switch:

A contact closure reed switch pulse output involves a magnet placed on a dial or drum that rotates or revolves as water passes through the meter. Each time the magnet passes near the reed switch, it is pulls the switch closed, then releases it as it moves away. Controllers, digital pulse counting displays, and other electronics count these pulses using electric current that they provide. The reed switch simply allows this current to pass from one wire to the other, which goes back to the controller or counter which then interprets the presence of current accordignly.