Leukoedema is a condition in which the inner lining of the mouth - typically the inside of the cheeks - takes on a whitish, grey-bluish appearance and becomes slightly swollen.
The 'benign lesions' were once thought to be a precursor to disease, but this idea is now largely rejected.
It's extremely common - some estimates suggesting that it affects as much as 50% of the population. It's so common in fact that it's now generally viewed as a 'normal' state rather than a medical condition.
The cause is unknown.
See : Wikipedia