Ok kiddo, do you know what a guitar is? A phorminx is like a guitar but it was played a long, long time ago in ancient Greece. Instead of having strings that you pluck with your fingers, a phorminx had strings made out of animal gut, which is the stuff that holds their insides together.
People would use a pick or a plectrum to strike the strings and make music. The phorminx was believed to be very important for singers because it helped them stay in tune and keep a steady rhythm.
Nowadays, we have lots of different kinds of instruments, but the phorminx was special because it was one of the first ever musical instruments!