For beginners, this is where you plug your guitar cable in from your amplifier

Inspect the threaded hex bolt, called a jam nut, within the socket. If it's loose, it will need to be tightened. A poorly fitted cable is the most common cause of any feedback you may experience when playing.

Another way to test the tightness of the jam nut is to insert your cable into the the jack and see if it's loose. Any movement while the cable is inserted can cause amp feedback.