How it works is in Wow Items get some items, mainly the high uality items get bound to your character so you cant give them to other caharcters, once the item is yours, its YOURS.
when doing a dungeon and getting loot that gets bound to the persron who recieves it, most guilds and groups disscuss who could benefit from the item the most and ether give it to that person or have the most eligible people roll for the item.
Ninjaing is while the rest of the party is discussing somebody who either is not in the running or didnt win the loot runs in and grabs the item so it is permanently bound to there character. so esstentially Ninjaing is stealing an important item right from under the nose of everyone else
people who ninja loot get know server wide generaly depending on the quality of loot they ninjaed and quite often have troube ever getting into a group again