It is here that you define your video signal sources. Typical sources might be Antenna (for an over-the-air signal captured by an antenna), Cable (for regular cable), or Cable-HD (for a high-definition cable signal).
Essentially, the sources defined herein describe all of the channels received on this source and the listings associated with them. If you are using a listings grabber (e.g. Schedules Direct or XMLTV), you can fill its information in here and pull the first block of listings from it. This will also set up the channels.
Otherwise, if you're not using a listings grabber or have other plans, you can just give your input source a name (e.g. Cable-HD). The channels can be set up in several other ways (see below).
Regardless of how you define your video sources, once you've done them once, you're pretty much set for life and can use the same sources over and over again for each video capture card that you define.