Can someone explain capping?

I've seen people wanting to run certain missions to cap an ability. This seems to me to have something to do with making an elite skill even better? Could someone explain the mechanics of this please?

capping - to cap an elite skill.
often people just want to do a mission to get to the boss having the specific elite skill and kill him. then they use their signet of capture for the elite skill and then leave. as easy as that. nothing special.

So cap = capture, and has nothing to do with raising the limit of a skill. OK, got it, thanks.

can't really raise the limit of the skill by means of altering the skill itself... but one can modify armor, or modify their weapon to raise the attribute of that skill to raise the limit of that skill. hope that makes sense.

Basically "cap" is short for Capture, thats what he was confused about, he thought "cap" might have meant there was a way to improve a skill (Like saying "capping off" something is filling it to the top).

But in Online games they shorten EVERYTHING because noone ever wants to write out an entire word