It is a proper fraction. An example of an improper fraction is 10/4 because the numerator is bigger than the denominator. An example of a mixed number is 4 5/7 So 3/7 is proper.
An improper fraction has a greater numerator than denominator, a mixed number has a whole number next to it. If the numerator is less than the denominator and there are no whole numbers next to it, the fraction is proper.