I'm assuming you're talking about the C-H vibration in alkenes vs. alkanes, as there are multiple vibrational modes in these types of molecules. So the wavenumber is just a measure of the energy required to stretch a particular bond, a quantity that's related to the force constant of that bond. So the analogy of a spring applies here. In physics, you learn that a spring is governed by some force constant that determines its stiffness. A stiffer a spring, the harder it is to stretch. A bond in a molecule is much like a spring. The stiffer the bond, the harder it is to stretch. Now, how does this relate to alkene vs. alkane C-H bonds? Well, alkene C-H bonds have an sp2-hybridized carbon bonding with hydrogen. In alkanes, the carbons are sp3-hybridized. You should know that the more s-character is in a bond, the stiffer that bond is. This results in more energy required to stretch it.
