Fats consist of fatty acids which are made up of long chains of hydrocarbons (chemicals that only contain carbon and hydrogen). Many naturally occuring fatty acids are unsaturated fats, meaning that there are carbon carbon double bonds in the hydrocarbon chains. These fats tend to be more oily in consistency due to these double bonds. Hydrogenation is the process of heating the fats in the presence of hydrogen gas under high heat and pressure to add hydrogen to the double bonds and convert them into single bonds. These fats are called saturated fats because the carbon chain is maximized with hydrogens (all carbon carbon single bonds). These saturated fats tend to have a more solid consistency. But if you only partially hydrogenate the fatty acids you get a partially hydrogenated fat that has the consistency of a semi-solid (like warm butter).