To answer the OP in better detail.

A wired cyclometer has a wheel magnet that passes a pickup placed near it on the fork. The pickup generates an electrical pulse every time the magnet passes it. That pulse travels up the wire to the cyclometer head where the pulse is counted and the interval between pulses timed. That information allows the cyclometer to calculate your speed and distance.

A wireless cyclometer has the same magnet except the pickup not only reads the magnetic pulse, it contains a small battery powered transmitter that sends that pulse as a radio signal. The cyclometer head has a receiver that reads that signal and does the same calculations as a wired cyclometer.

The tricky part is the sending unit and cyclometer head have to be aligned properly so the head "sees" the signal. Also, there are two batteries to be concerned with as both the sending unit and receiver have their own. And as noted, outside radio frequency sources can interrupt or interfer with the signal.