This button will appear if the source is set to either Mic/Line in, or both Mac audio and Mic/Line in. Clicking on the Configure button will bring up the Audio MIDI setup utility. In most cases, for most users, the input device will never have to be configured.
However, if you find that recordings made from a line in are of poor quality, but all of the settings in WireTap Pro are correct, click on the Configure button. Change the Sample rate of the input device in the Format field of the Audio MIDI Setup utility.