The F1 - F3 keys are used to hot swap your weapons on the fly. Charges gained on a weapon are retained until used, even if you change to a different weapon. That opens up the possibility to charge all three equipped weapons then discharge them all in a single fight.
Each class has weapons that are native to their class but you can equip any weapon to take advantage of the specific skills available through each weapon type. This feature opens up a lot of unique combinations and more interesting game play.
See the User Interface page for more information on available hot keys.