The carat is the unit of weight of the diamond. Depending on the number of carats, you know the approximate size of your diamond. But this criterion alone does not determine the size of your diamond or its value, since the final result will also depend on the cut of the diamond. This means that a very well cut diamond with a small carat may be worth more than a diamond with a higher carat but which would be less well cut.
It is therefore important to combine the carat weight with the other criteria of the 4Cs.