Torque and Horsepower are two ways of describing the same general concept: the ability to move something. Both are used to describe the output of a motor vehicle and the engine in it. But, they represent two very different ideas. 

Horsepower is generally understood as the ability to pull in a straight line at a sustained speed, like the horses James Watt chose to illustrate the concept in the 18th century. Torque, on the other hand, is rotational force that overcomes inertia to accelerate a load from rest, at an increasingly greater pace. In the context of a pickup truck like the Chevy Silverado or GMC Sierra, horsepower indicates the top end (on a flat or inclined surface) it can achieve doing work. Torque refers to how quickly the truck can build speed, from a slower speed or standing start.  

While both trucks can do an admirable job of moving a mass that, as Newton observed, would always prefer to stay in one place, torque and horsepower are simply two important ways of measuring the application of energy to motion. The 305 foot-pounds of Torque (in the stock 4.3 liter 6-cylinder engine) tells you how quickly the truck breaks its inertia and accelerates from a standing stop, or pulls into the passing lane. The 285 Horsepower gives an idea of the relative top speed (or degree of incline it can climb at sustained speed) or payload.

Both are nice. One is fun, the other is utilitarian. For obvious reasons, manufacturers try to create a balance between the two. Otherwise you would find one vehicle that accelerates quickly but tops out at 23 mph. And another that burns a lot of fuel making lots of noise and heat but needs a long downhill stretch before it can flex its muscle hauling a load.