Window Regulator
 

Anyone know why the little spring would keep jetting out when the window is down? Any idea what it does? I'm afraid it's going to smash the window because when it happens I hear a bang.. then it falls inside the bottom of the door.

As far as I know, the spring is likely part of the window mechanism, helping it move smoothly. If it's shooting out like that, it could cause damage if it gets caught in the wrong place. I think it's safer to avoid using the window until an expert checks it and fix it.

Electric or manual? Dorman window regulators are inexpensive.
What year truck?

There is usually a spring on the window mechanism itself which helps with the "lifting" of the window back up....if it's popping off and coming loose than the regulator is likely broken or worn out so badly the spring will no longer stay in place. Sounds like it's time for a new regulator assembly.