An irreducible fraction is a fractiona / b such that there is not another fraction c / d with c < a and d < b that represents the same number. Note that all numbers in a fraction are integers.
For example the fraction 2/4 is equal to 1/2 and therefore not irreducible, but the fractions 1/4, 5/6 and 101/100 are irreducible.
It can be shown that a fraction a / b is irreducible if, and only if, a and b do not have a greatest common divisor.