Tagging is when you attack an npc and then go away to do other stuff, as it is proper manners to leave an NPC that already has been attacked alone to whoever who attacked him unless you have been given permission to finish the NPC. (This is usually done during bootrushing)

However, there are varying opinions on the validity of a tag.  Some (especially older) players will only respect a tag when an actual fight has taken place, and frown upon the practice of attacking an npc and leaving just so no one else can get the equipment.  This has lead to attempted and successful pk’s in the past.

see also MD Manners