$H_2$ is added to an alkene in the presence of platinum catalysts, palladium or nickel/aluminum alloy.
Many plants such as sunflower, rapeseed or olive contain so-called "vegetable" oils
These oils are esters of glycerol (propane-1,2,3-triol) with unsaturated fatty acids, for example:
Vegetable oils are viscous liquids, which is an economic disadvantage. To overcome this disadvantage, commonly practice is hydrogenation to turn them into less unsaturated margarines which are spreadable:
Following the heating needed to hydrogenation, the typical "cis" configuration of double bonds can turn into "trans" configuration ...
The "trans" fats so obtained are sworn enemies of many dieticians!