Payload is an important number as if you are not careful, you can exceed the weight the car/truck was meant to CARRY. Think of it as how much stuff you can load into your car without overloading the suspension, etc...
I was one of those guys that thought "oh, my suburban with 8600lb tow rating can easity pulll our #6500lb RV." 20 min down the road I knew I was in trouble. Suburban felt like it had two flat tires. Sadly, it only had a pitiful 1425lb payload capacity.
1425lbs Rating
-600lb hitch
-700lbs for 2 adults, 2 teenagers, and 2 kids
-150lbs for 2 kayaks
-??? for 6 bikes
-??? for etc
you can see you can get into trouble quickly with a 1/2 truck. Our 3/4 ton truck sits happy with 2300lb payload capacity.