Number which can be expressed as in the form of p/q where q != 0 (q is not equal to zero) are called rational numbers.
Obviously numbers which cant be represented in the p/q form are called as irrational numbers.
for example 5.3 is rational number because it can be represented as 53/10.
but 4.12658243591973467..... can't be represented in p/q form. Hence it is irrational number.