Unfortunately, the tag has no tag info/wiki. Does it refer to a particular thing in D&D 4e specifically? I'm guessing it does, and I'm guessing that is what differentiates it from the general terminology tag.
Now nine; I retagged Where can I learn the meanings of RPG terminology? and What is fighting? to not use this tag.
It looks like the tag got started in D&D 4e. Keywords in that game are a specific element of power definitions: Arcane, Implement, Fire, Weapon, Healing, Primal, etc are keywords. Keywords can reference a myriad of things: power source, damage type, power category, whether you need to use a weapon/implement to use it, etc. They're a significant element of character building because a lot of things care about specific keywords. If you have an Arcane build, you'd focus on picking up Arcane powers and on finding feats and items which apply the Arcane keyword to otherwise non-Arcane powers. For example see Is there a way to add the "primal" keyword to a power? or How can I add psychic damage to wizard powers?.
I have added a tag definition:
for questions about managing the keywords of D&D 4e powers
That's a very specific definition for a very general concept, but it reflects usage right now.