An effects unit is a magical box that can change how music sounds. Kind of like how you put different stickers on your drawings to make them look different, an effects unit can add different special sounds to music.
There are many different types of effects units, but some common ones include pedals that guitar players use to make their guitar sound like it's on fire or underwater. There are also units that DJs use to make funky sounds or echo effects.
Each effects unit has its own special way of changing sound. Some effects units can make music sound like it's coming from a big room, or a tiny closet, or even from outer space!
Overall, an effects unit is like a treasure chest full of sparkly sounds that musicians use to make music sound more interesting and exciting.