MapInfo serves as a methods of customizing some asthetic properties of a map which would otherwise be unchangeable, as well as adding the ability to use some Eternity-specific features. MapInfo is used by editing the map's header in a wad editor with the "View raw data" (XWE) option turned on. A basic MapInfo setup may be similar to this:
[level info] levelname = Evil Hell Lab creator = DooMGuy667
This would change the automap and intermission screen names for your map to "Evil Hell Lab" and when the creator command is used in the console, output "DooMGuy667".
Alternatively, an EMAPINFO lump may be used to change MapInfo variables on multiple maps. For example, create a lump in your wad named EMAPINFO and add this to it:
[E1M1] levelname = NotHangar creator = NotRomero
This will change the MapInfo properties for E1M1. The name in the braces is the map the MapInfo applies to, so by changing [E1M1] to [E1M2], the changes would apply to E1M2 instead. Note that MapInfo information in a map's header will be used instead of the information in an EMAPINFO lump, when available.