Warm air contains water vapor which is invisible, and warm air is less dense so it tends to rise. As the air rises, the temperature and pressure decreases. This progressively lowers the capacity of the rising air to hold water vapor until a point is reached (called the dew point) at a certain altitude where the water vapor in the air transitions to the liquid state forming minuscule water droplets. If enough of these water droplets are present, the water in the air will not be invisible anymore and will acquire a white color forming what we call a cloud. If the water droplets become larger, the cloud acquires a grey color. The air can continue to rise pushing the top of clouds to great altitudes and making them look fluffy with a lot of dome-like bumps.
If the layer of air below the clouds is free of turbulence, the point at which the transition from water vapor to water droplets occurs will be located at more or less the same altitude over a large area of land, Because of this, all the clouds over this area will seem to have a more or less uniform flat bottom located at the same altitude. In the video below, you can see many clouds with fluffy tops which have flat bottoms located at roughly the same altitude, as though all of them were resting on an even surface.