When you buy a truck with a tow package it means that the truck will be beefed up and have a heavier duty engine, transmission, transmission cooler, beefed up springs and leaf springs etc.
The tow package also includes the wiring needed to tow your trailer such as a 5th wheel trailer wiring and a trailer hitch installed under the truck.
A truck with a tow package will also mean that they design the engine, transmission, brakes, frame, electrical system, and cooling system to handle the extra weight of the towed load.
You can install a trailer hitch under your vehicle to allow you to tow a trailer or install a 5th wheel trailer hitch in the bed of your truck and install the wiring yourself or allow a trailer place to install the trailer hitch and wiring for towing.
If you do a lot of towing with your vehicle and have an automatic transmission you should install an additional transmission cooler to help keep your transmission running cool and extend transmission life.