What is ninjaing?

From our own Glossary on in-game terms (which always stirs up the natives when posted) there is a simplistic description:

http://www.worldofwarcraft.com/info/basics/glossary.html

Ninja - To try to loot an item without other players knowing or paying attention. Basically, to take an item without permission.

If you're unsure of what you're supposed to be doing when rolling for something, just ask. If you don't need the item, it's usually ok to roll greed on it. In some cases where an item is bind on pickup groups will pass if the item isn't needed so they can disenchant it and then roll on the shard.

In any case it's best to ask or when starting any instance ask what the looting rules are for that group. It can vary for specific items from dungeon to dungeon, or even group to group.