Check the weight carrying (not towing) capacity on the sticker on the side of the driver's door.
On the one shown below, it indicates that the total weight of passengers, cargo and TRAILER TONGUE WEIGHT should not exceed 1607 lbs. This is also known as the "payload". It also indicates that there should not be more than 3850 lbs. on the rear axle or 3450 lbs. on the front axle or 7100 lbs. on the whole truck.
When the trailer tongue is set down on that hitch ball behind the rear truck axle, it will lift the front of the truck some. With less weight on the front tires, it can cause a loss of traction and less capability of the steering and braking to work properly. The purpose of the weight distribution hitch is to return some of that weight to the front. One method to determine its effect is to measure the height of the front fender well before and after the trailer is attached to determine how far it rises. The weight distribution hitch tension bars should be cinched up to provide for the fender to be lowered at least half way back to its unloaded height. If you buy a new weight distribution hitch (WDH), you should get some installation instructions. If you get a used one, you should go to the manufacturer's web site and download the instructions.
My truck has a 1607 lb. "payload". With a trailer tongue weight of 900 lbs. and only the driver on board, the rear axle and the whole truck are loaded to 94% of their maximum weight.
Note that the WDH returned 72% of the weight that was lifted from the front axle.