I know this will sound like a boring (or condescending) chemistry lesson so I apologise in advance. Basically fats (and oils) contain molecules made up of three hydrocarbon chains ie chains of carbon atoms with hydrogen atoms attached. In a saturated fat all of the carbon atoms are joined by single bonds. In a mono-unsaturated fat two of the carbon atoms are joined by a double bond. In poly-unsaturated fats several of the carbon atoms are joined by double bonds. Now, don't worry about the difference between the bonds, the more double bonds the more likely the fat (oil) is to be a liquid at room temperature, ie an oil, and there is evidence to suggest that the more double bonds the less likely the link with heart disease. In a nutshell, the higher the percentage of polyunsaturates then the less damage you are doing to your heart etc - probably.