Cars and trucks all have that rating, as you’ve noticed. So, if your truck weighed 3300 lbs (curb weight) and you climb in (say 200 lbs) you can carry another 1500 lbs total… to reach GVW. But it needs to be balanced because each axle has its own rating.
If you have the owner’s manual, Ford has about 15 pages in the book explaining about weight loading. The first page gives the gist, telling how much weight can go in the bed, followed by pages of confusing data, how much you can carry when towing a certain weight, etc. It’s best to place most of the load at the rear of the wheel wells and forward, when possible.
On my Ford half ton, the factory hauling capacity is nearly 3/4 ton, and a towing capacity of 5850 pounds… but it’s one or the other. I know people who’ll toss in over 1/2 ton, then hook a 4500 pound trailer… and the truck is overloaded. Stick to the book and your drive will be safe.
I hope that makes sense. If you don’t have the manual, most manufacturers have them available for download.