A banjo is a musical instrument that looks a bit like a circle. Imagine a big wooden hoop, like a hula hoop, but smaller. This is called the round body of the banjo. On top of this round body, there is a thin piece stretched really tight, like when you pull on a balloon to make it round.
This thin piece is what makes the sound when we play the banjo. When you hit or pluck that thin piece (which is really the string), it makes beautiful music! So, the round body holds everything together and helps the banjo make sounds that we can sing along to.