Dear Student,
Rancidification is the process by which fats and oils (or lipids in general) undergo decomposition due to oxidation or hydrolysis. Rancidity of fat/oil can be reduced by keeping it in a cool, dark place with little supply of oxygen. Also as student Sanchit suggested in the comments section, the rate of rancidity decreases with temperature.
Thus the refrigerator proves to be an ideal storage to place oil or fat products so that it doesn't become rancid.
Regards