What does it mean for a number to be irrational?

An irrational number cannot be expressed as a ratio between two numbers and it cannot be written as a simple fraction because there is not a finite number of numbers when written as a decimal. Instead, the numbers in the decimal would go on forever, without repeating.

And irrational number is a number than cannot be written as a simple fraction.

Two good examples of irrational numbers are PI and the square root of 2. These numbers have never-ending decimals, and their decimals never repeat.