A fuel sending unit is what sends the signal to your gas gauge on your dash, letting you know how much fuel is in your tank. It is made up of a float, similar to what you would find in your toilet tank, connected to a thin metal rod and an variable resistor. A resistor restricts the flow of electrical current. When your tank is full, the float is at the top, and the resistor is allowing most of the current to flow through, which translates to your fuel gauge showing "full." When the float moves down, as your fuel level drops, the arm moves the resistor, allowing less current to pass through. As less and less current passes through, your fuel gauge shows less and less fuel in your tank.
Depending on the year of your Jeep, the fuel sending unit could be a stand alone unit, or it could be integrated with and in tank fuel pump and strainer. Stand alone types are usually found on older Wranglers and CJ's, while most newer Jeeps use an in tank pump and sending unit.