The reduction of alkenes is simply breaking the double bond and forming two new C-H bonds. The product is an alkane.
The reagents required for this reaction are Hydrogen H2 and a metal catalyst. There are two common catalysts used, Platinum PtO2 or Palladium with Carbon Pd/C. This reaction is called Catalytic Hydrogenation.
The H2 bond is broken and is adsorbed onto the catalytic surface. Similarly the alkene attaches to the metal catalyst.
The hydrogenation of the alkene then proceeds whilst attached to the metal surface. Once both carbons have formed a bond with a hydrogen atom, the product detaches from the catalyst surface.