The editor allows players to "edit" / create their own maps for us in MP.
You can edit the terrain, insert objects, buildings, game functions(spawn, flag spawn and so on) and create your own maps from an empty template. In the past people have created very strange and cool stuff.
Exactly what Oey666 said. Instead of having 12-15 multiplayer maps like COD/Halo and then a few DLC packs, we get infinite maps because people like me will ALWAYS be making new maps. Even when the multiplayer community dies.
Arrow in the knee: http://skyrimnexus.com/imageshare/image.php?id=10753 SPACEngarde: http://skyrim.nexusmods.com/imageshare/image.php?id=41978
I hope the online community will last longer this time.... FarCry 2 lasted maybe 3 months, and it wasn't all that fun with such HUGE maps.
The main reason for that was the multiplayer in general being very bland. No matter what map you made, people still played the same way. It took away a lot of creativity from map makers. To fix that, we either need weapon placement back, or a way to make very detailed gametypes(or preferably both).