What is Automatic Activation in WCF?

Automatic Activation means service will start\activate when a message request is received. In IIS, service is not always running. Service is monitored by ISS itself.  IIS check if service is idle or unresponsive then it stops the service process. The next time a client calls the service, the process is recycled and restarted.

Note:-

IIS and WAS provides Automatic Activation.

Reference:- MSDN