The quickslots are the twelve little slots above the character portraits in the main view. Items, spells and abilities can be stored here and accessed via the F1-F12 keys. To store an item from the backpack in a slot, click the "kettle" button at the bottom of the character window, then click the desired item, then the desired slot. Spells and abilities are stored similarly, except that you use the kettle button at the upper left.