A tileset is the combination of tiles in a level or map. Some maps are created with only the tileset so that others can experiment with objects without working on a tileset.
NUMA-goers are very impressed with tilesets that look fancy or professional. In fact, one important part that people look for in a map is the tileset. With a better tileset, it will be more likely to attract someone's attention when they look at the thumbnail image on NUMA.
See more about tilesets in this article: mapmaking.