The valve is installed in the linner. It will let water or gas that is below the liner in so the winner will not be bushed to the surface. When a liner is allowed to be bushed up it looks like a whale in the water.
The valve looks kind of like a flap that will open when pressure is applied but will not let water out.

You may not need this. Check with the people who sell the liner tell then your situation and then make the decision.

THIS MAY NOT BE CORRECT BUT THIS VALVE COST LESS THAN 100 DOLLARS

If the liner company does not know about this get a second opinion

OTTO