Typically, a servlet class is instantiated the first time it is invoked. The same instance will be used over several client requests, so all members that are declared in that servlet are shared across clients. That is What is meant by multi threaded model, multiple clients that access the same instance.

There are situations where you want to protect your servlet member variables from being modified by different clients. In this case, you can have your servlet implement the marker interface SingleThreadModel. Every time a client makes a request to a servlet that implements this interface, the engine will create a new instance of the servlet. For performance reasons, the engine can also maintain an instance pool, handing out instances as they are needed. Or it could also serialize client requests, executing one after another.