Ranged weapons that loose projectiles, including bows, crossbows, and slings, take some time to load. When a weapon shows “load free” on the weapon table, that means you draw and load ammunition as a free action, effectively part of the action used to attack with the weapon. Any weapon that has the load property requires two hands to load, even if you can use only one hand to attack with it. (The sling, for example, is a one-handed weapon, but you need a free hand to load it.) The crossbow is “load minor,” which means it requires a minor action to load a bolt into the weapon. If a power allows you to hit multiple targets, the additional load time is accounted for in the power.
Published in Player's Handbook, page(s) 217.