The clutch is a thing to prevent the chain coming off the front chainwheel under extreme vibration in off-road conditions.
In a normal road mech, one purpose of the arm on the rear mech is to act as a chain tensioner, to take up the slack as the chain length required changes as you shift gears.
It is spring-loaded, and gives and takes chain length readily.
Off-road, this does not work so well. As the bike bounces around, the arm will flap forward and back easily. As it flaps forward, the lower run of chain will become slack, increasing the risk of the chain dropping off the front chainring.
The clutch is a friction device on the rear mech.
It will not allow the arm to move forward easily.
So the arm will not bounce forward in response to a bump, and create slack chain.
But it will move forward in response to the force created by a gear change.
The clutch only operates on forward motion of the arm: it will resist giving out slack, but it will move backwards taking up slack instantly.
It has no real purpose on road mechs.
It can be dis-engaged using a lever on the mech.