Nucleophillic addition reaction in the presence of an acid catalyst on a molecule with an alkene
The acid will protonate the O of the carbonyl carbon, so a positive charge is generated. The oxygen of the alcohol with the free pair of electrons will act as nucleophile. So the alcohol is added to the cycle and a pair of electrons of the double bond of the carbonyl will compensate the positive charge generated. As the medium is acid, in the second step the hydroxyl group is protonated, so it is generated a water molecule, the alcohol attacks again at the same carbon and the water is released to the medium.

This is why this process is used to protect carbonyl groups. The reaction is reversible so if you add water, the reaction direction is reversed and you get the keto form again.