Here is a list:

Dry Weight/Shipped Weight - The weight of the RV as shipped from the manufacturer without any passengers, cargo, liquids, or additional accessories or dealer installed options.
Gross Vehicle Weight (GVW) - The current combined weight of the RV (dry weight) plus passengers, cargo, and liquids.
Gross Vehicle Weight Rating (GVWR) - The maximum allowed weight of an Vehicle/RV, including passengers, cargo, fluids, etc..
Cargo Weight - The total weight of all passengers, food, clothing, liquids, accessories, etc.. that you might put into your RV.
Tongue/Hitch Weight - The weight/pressure applied from the trailer tongue/coupler to the conventional/bumper hitch on your vehicle.
Pin Weight - This is often referred to as the weight/pressure applied from fifth wheel king pin to the hitch in the bed of your truck.
Payload Capacity - For Trucks the amount of weight you can safely add to the bed of the truck. Your Fifth Wheels Hitch Weight or Pin Weight plus the actual weight of hitch plus any other accessories you have in the bed of the truck must not exceed the Payload Capacity amount. Confirm Payload Capacity with your vehicle’s manufacturer.
Max Towing Capacity - The maximum weight limit that can safely be towed by your specific vehicle. This weight is calculated by adding the RV’s GVW with weight of all passengers, cargo, and liquids in your tow vehicle.

You are probably referring to the last one. The Max towing capacity.

Your Ranger's max towing capacity is based on the weakest link of lots of different components that include: Axles, suspension, transmission, hitch, differential, engine output, tires, brakes, and other components. It says you can tow UP TO that much weight, BUT you still have to tow it the right way and you might not be able to tow it very fast or up steep grades very well. You also have to add the weight of the vehicle, cargo, passengers, fuel and trailer together to also verify that you don't exceed the GVWR. (so even if you can tow up to 7500 lbs, you might be well over your GVWR if you have passengers and cargo loaded into the truck, even if the trailer is under 7500 lbs). Take special care to make sure that your tongue weight is about 10-15% of your trailer weight. Equalizer hitches can make all the difference in a white knuckle trip vs a pleasant family outing and really do work.

That was the two bit answer.

The nickel answer is that I probably would not get a trailer with a dry weight over 5K if you are rated to 7.5K. The performance will not be pleasant and I know of lots of people who buy trailers right at their trucks weight limits only to sell them and get something smaller so they don't feel like a turtle in the fast lane listening to their engine sound like it is going to explode.

hope it helps