Acceptor threads are threads that wait for connections. The threads accept connections and put them in a queue where they are then picked up by worker threads. For more information, see Connection-Handling Overview.
Ideally, you want to have enough acceptor threads so that there is always one available when a user needs one, but few enough so that they do not burden the system. A good rule is to have one acceptor thread per CPU on your system. You can increase this value to about double the number of CPUs if you find indications of TCP/IP listen queue overruns.
This setting is tunable when you edit an HTTP listener. The number of acceptor threads defaults to the number of CPUs on your system.
Other HTTP listener settings that affect performance are the size of the send buffer and receive buffer. For more information regarding these buffers, see your operating system documentation.