Bills right, you need to go and read about servlets.
But in a nutshell, it essentially allows you to retrieve configuration parameters from the web.xml file, for your particular servlet. (Each servlet in a web app can be initialized differently). Hence the name, "Servlet Configuration" shortened to ServletConfig.
As opposed to the ServletContext interface, which really represents the servlet container to any servlet in the web app. The object that implements the ServletContext will be the same for all servlets within a web app, and so all servlets can get a copy of it and it will be the same for all of them.
I would recommend "SCWCD Exam Study Guide" by Deshmukh and Malavia. It's a classic now for studying for the SCWCD exam, but it's very useful for general brushing up on servlets and JSPs.