 When someone talks about an organic molecule being saturated or unsaturated, it's a way of classifying whether or not that molecule has double or triple bonds or only single bonds. A saturated molecule is one that has only single bonds, while an unsaturated molecule has one or more multiple bonds, double or triple. The reasoning behind these terms is this. When a hydrocarbon has all single bonds, it's carrying the maximum number of hydrogen atoms that it's possible for that molecule to have. Let me draw out pentane, C5H12. Now there is no way to attach more hydrogen atoms to that molecule, all possible bonds have been used, so we say that the molecule is saturated with hydrogen. However, let's draw it with a double bond between two of the carbon atoms, so it becomes pentene. In order to form this double bond, we need to lose a hydrogen from each of those two carbons in order to free up two electrons to form the new double bond. So the molecular formula for this compound is C5H10. It's therefore called unsaturated because it would be possible by breaking this second bond here to attach more hydrogens to this molecule.