Like the name says
The vent solenoid allows the tank to vent when the vehicle is parked. When power is off gas vapors from expansion go through the canister and are absorbed. This valve is open when the power is off. When the engine is running, the purge valve opens and gas vapors are sucked into the intake manifold and the canister is cleansed of gas vapors. This is an over simplification. There is also a tank vapor pressure transducer that allows the computer to monitor this. If it doesn't see a change when a valve is opened, you get a light. The vent solenoid on the canister is prone to rusting that can cause the valve to stick. If it looks bad, it probably is. These problems can be difficult to diagnose. There is a tap under the hood near the battery that allows the dealer to pressuriize the system and diagnose. The vent solenoid is $36 which is cheaper than a dealer will charge to tell you what is wrong.
