Photocell are sensors that detect light. When they are used with barrier gates, they work by triggering the gate to open or close when a certain level of light is detected. 

Here's how it works:

1. When the photocell is exposed to light, it generates an electrical signal.

2. This signal is sent to the control unit of the barrier gate.

3. The control unit then activates the motor or mechanism that opens or closes the gate.

4. When the light intensity drops below a certain threshold, the photocell stops generating the electrical signal and the control unit deactivates the motor, causing the gate to close or remain closed.

This system is often used in parking garages, and other locations where barrier gates are needed to control access or provide security. By using photocells, the gate can be opened or closed automatically based on the amount of light detected, which can save time and reduce the need for manual intervention.