The terms nerf and nerfing refer to the editing of a challenge to make it easier so that more players will be able to beat it. While the challenge was already perfectly beatable before being nerfed, the map creators usually wish to balance the difficulty in a map and will edit it anyway to make it easier. This term was first used extensively by the EX Crew since many of their challenges had to be nerfed to prevent too many players from quitting the map before reaching the end.
The term comes from the NERF brand of Hasbro toys, namely the blasters, which use rubber-tipped foam darts instead of bullets/paintballs/BBs etc. as a safety measure.