TP are your tactical points. TP are required to use your weaponskills. As you use weaponskills, your TP will decrease. You cannot use any weaponskills if your TP reach 0.
If the TP cost of a weaponskill is greater than your remaining TP, you cannot use that weaponskill. TP slowly regenerates over time in and out of combat. As of patch 5.0, TP no longer exists. Weaponskills now have no cost.