To substract two fractions, we have to have common denominators.
For example, if we want to substract 1/3 from 1/2, we have to convert the fractions so they have a common denominator of 6.
The denominators are the ones that give sense to the fraction. In fact, when we substract whole numbers, we can consider we are substracting "fractions of one".
Then, for example, 5 - 4 could have been written as 5/1 -4/1.
If the numbers have different denominators, we can not perform additions or substractions until we express the fractions with the same denominator.
Trying to substract two fractions with different denominators is like substracting two things that are not expressed in the same units and it will yield an incorrect result.