Nerf is an informal game term. Nerfing something is to reduce its strenght, usually on a card that is broken or enabling a loop. Errata issued by Decipher were nerfing errata, most infamously with Frenzy of Arrows (18U79), which was nerfed into complete uselessness (there are no Orc archers). An entire strategy can be nerfed, as the fan group Players' Council has attempted to do with some of their errata and Vsets.
The inverse of nerfing is buffing.