Nitrogen is used in shock absorbers because of its inert quality. It remains a gas under high pressure and doesn't corrode the inside of the shock. Regular compressed air contains moisture which would slowing destroy the shock from within. If the shock losses its pressure for an unknown reason you will most likely have to rebuild or replace it. You should never need to recharge a shock unless you have just rebuilt it, but filling the shock with new Nitrogen is a simple process if you own the right equipment.
Install the hose and gauge on the tank by threading it into the tank's receiver.
Attach the filling coupler onto the shock's filling valve.
Turn the valve on the top of the Nitrogen tank releasing the gas.
Monitor the gauge until the desired pressure has been reached and close the tank's valve.