Treasure is random items you find scattered around the dungeon map, or left behind by defeated monsters. It can range from almost useless trash (a rat skeleton), to useful or magical items, or even just gold coins. It may be hidden or locked away in chests or behind traps. Items may be unknown until properly identified, and reveal themselves to be terribly cursed (a cursed helm of blindness) or unexpectedly blessed (a rapier of demon-slaying).
You may need to carry promising bits back to town level to sell or appraise.
A singular game winning treasure could be e.g. retrieving Rodney's amulet of Yendor.