Io moves in an elliptical orbit around Jupiter. It moves fastest when it's closest to Jupiter, and slowest when its far away. But it spins on its axis at a constant speed, so rotation sometimes gains on orbital speed and sometimes drops behind. Jupiter tries to stop the shaking with its mighty gravitational grip. That makes rocks inside Io rub together and heat up, melting sulfur and other elements.
Sulfur compounds jet from the volcanoes.
Sometimes material blasted from the volcanoes escapes Io entirely. For example, sulfur from Io colors its neighboring moon Amalthea bright red. Sulfur on Io assumes different colors depending on the specific chemical compound and on its temperature. Black, white, and even blue sulfur stains the surface. But mostly Io is orange and yellow.