Hydrogenation is a process that will add more hydrogen atoms to a molecule.
Saturated hydrocarbon chains contain only single bonds between carbon atoms. This allows for a maximization of the number of hydrogen atoms in the molecule. Remember that every carbon will form 4 bonds. A carbon in the middle of a chain (...C-C-C...) forms bonds with neighboring carbons and can form two added bonded to hydrogen atoms.
An unsaturated hydrocarbon chain will have carbon atoms joined by double bonds (...C-C=C-C...). In this chain the second and third carbon are already forming three bonds, so each of them could only bond to one atom of hydrogen. Hydrogenation would add a second hydrogen atom to both the second and third C of this hydrocarbon chain.
Hydrogenation of an unsaturated lipid (oil = liquid) will change its properties to those of saturated fats (fat = solid). This is how margarine is made.