When there is one carbon-carbon double bond within the fatty acid chain, the fatty acid is described as being monounsaturated : the occurrence of two or more carbon-carbon double bonds in the fatty acid chain will result in a polyunsaturated fatty acid.
The double bonds mean that the carbon atoms have fewer hydrogen atoms attached to them. Fats formed from this type of fatty acid will be softer. The greater the degree of unsaturation, the softer the fat and, indeed, an oil may be produced. Softer fats are naturally more susceptible to oxidation.