Now look at the triangular numbers. So numbers like 12 are called composite. First we need to know what an integer is. It is just a positive or negative whole number. A number is a rational if we can write it as a fraction where the numerator and denominator are both integers.
Every integer is a rational number.
A irrational number is any real number that is not rational. Meaning, a number that we can conceive of in this world, one with no square roots of negative numbers. A real number is a number that is somewhere on your number line. So, any number on the number line that isn't a rational number is irrational. For example, the square root of 2 is an irrational number because it can't be written as a ratio of two integers.