Clouds are formed by rising water vapour.
As it climbs, that invisible water vapour cools and condenses into tiny, visible water droplets and that's a cloud.
Within that cloud the air is moving around in complex and unpredictable patterns of updrafts and downdrafts that are more dense than the surrounding air.
Fleecy white cumulus clouds, for example, one of the most common types, look like a bunch of bubbles all stuck together to form a mass, and those bubbles reflect the differing pockets of air inside the cloud.
An aircraft responds to those updrafts and downdrafts by experiencing greater or lesser lift on its wings, and that differential is what causes turbulence, the juddering and bumping that passengers feel inside the aircraft.
Pilots seek to avoid clouds in favour of clear blue sky but that's not always possible.
Although it might feel like the aircraft is shaking itself to bits, the actual vertical and lateral movement expressed in metres is fairly small, and far short of what would cause an airframe to experience catastrophic failure.