As a non music person, I really do not know what it is until I read an article regarding it in a music magazine. Pedals or effect units are actually devices that change the sound of musical instruments or audio sounds.
Musicians usually use this electronic device during live performances or studio recording to put color or dramatic effects in the sound. This is often used with electronic or electric instruments such as the electric guitar, electric piano or electronic keyboard, etc.
So if you see devices mounted on a pedal train, like those above, don’t be surprised to its complexity, they are called pedals for the musicians.