A ballgag is a gag used in BDSM with a rubber or silicone sphere strapped into the mouth behind the teeth.
Ballgags are items that some people use for sexual purposes. To make a makeshift ballgag is to use some type of clothing like a sock, scarf or sheets.
There are several ways you can make a ball gag. My favorite is good for self bondage, espiacally when your hands are bound. You core an apple, then run string or rope or shoelaces through the hole. Bite down on the apple, then tie the string(etc...) behind your head. Viola! If you want, to make breathing easier, poke holes through the apple. This makes it so you know you won't suffocate. Yay!