Generally, a spoiler is anything which reveals an ending or ruins a surprise. In the NetHack sense, a spoiler is a file documenting the behaviour of the game in great detail. Hundreds of different spoilers exist, from tables of weapons and monsters, to FAQs, to comprehensive guides. Many are now out of date; be sure to check which version of NetHack the spoiler was written for.
Wikihack articles, collectively, constitute a spoiler.
Contrary to popular belief, it is possible to ascend without consulting spoilers. The Guidebook is generous if you pay close enough attention, The Oracle provides clues to the more arcane aspects of the game, and sheer bloody-minded persistence will see you through eventually. The first well-documented instance of a totally unspoiled player ascending is in this RGRN post.
See also: Category:Spoiler files