if you are giving ocewcd 6 exam then you have to learn pretty much about AsyncContext. i would recommend you to check the oracle offical site regarding exam topics and there is mention of Asynchronous processing in servlet 3.0 . if you are giving the older exam i.e. 5 then you need not learn about it.
there is topic about Asynchronous processing in servlet 3.0. that is what means everthing relating to AsyncContext interface. in short implementatino of AsyncContext is the execution context when you start asynchronous processing.