This is the lowest/simplest ratio of the molecule. For example propene has the formula C3H6 - if we look a the subscript numbers (the ones down here!) we can see that smaller numbers are possible. It is important to know the relationship between these numbers. In this case you can write this simply as CnH2n. Therefore the empirical formula for propene is CH2.Another example is propane which is C3H8. We can work out that the relationship is as follows: CnH2n+2 and therefore the empirical formula is CH4.