Form I-539 is used by some nonimmigrants to extend or change their nonimmigrant status in the United States.
Nonimmigrants are otherwise known as temporary residents and are given visas based on what they intend to do in the United States, whether it is to study or vacation.
Form I-539, Application to Extend/Change Nonimmigrant Status, lets these temporary residents either stay longer on their current visa designation or to switch to another visa designation.
There are other, more complicated purposes for this form that you can learn more about in the form instructions.