A dimension can be a driving dimension or a driven dimension. 

A driving dimension controls the size, location, or orientation of an element.

If you edit a driving dimension, your design will automatically change to reflect the new value.

A driven dimension measures the size of an element, but it cannot control the size. You cannot edit a driven dimension directly.

A driven dimension depends on a driving dimension for its value.  If you edit a driving dimension, the associated driven dimensions will also change.