It is a device that monitors the flow rate of water passing through a pipe and shuts off the system if the flow falls below a certain level.
This is important because a decrease in flow rate can indicate a clogged or dirty filter, which can cause damage to the pool pump or other equipment.
The flow cut off switch is typically installed on the pipe leading from the pool pump to the filter, and it is connected to the pool's electrical system. When the flow rate drops below the preset level, the switch will activate and shut off the pump, preventing any further damage to the equipment.