Rational numbers are numbers that can be written as a fraction. Note that another name for fraction is ratio which is the root of the word Rational. Almost all of the numbers you can think of are rational. All integers, all terminating decimal numbers, and all repeating decimal numbers name a few..
Examples: -4, 3.5,

Now you may have already guessed Irrational numbers are numbers that we cannot write as a ratio(fraction). Fortunately, this is a much smaller pool of numbers.. Basically, these are non-terminating and non-repeating decimal numbers.. In other words, decimal numbers that go on forever without ending and never repeating or displaying a pattern.
Where do these types of decimal numbers come from? I would suggest you consider the following two possibilities for now:
1. the number

2. anytime you have a non perfect square number under the square root symbol.... i.e.

Now this is also true for any other radical numbers with different indices.