A rational number can always be expressed as the quotient of two integers (whole numbers). For example:
3=3/1, ½, 1/7, 49/123, etc. Irrationals cannot be expressed in this way, so, for example, √2, ∛10, 50.25 can't be expressed as the quotient of two integers. π and e (base of natural logarithms) are also irrational.