A decimal separator is like a little dot that helps us separate the whole numbers from the tiny parts of a number. For example, if we have the number 3.14, the dot (decimal separator) helps us know that the "3" is a whole number and the "14" is a teeny, tiny part of a number.
We know that we use different symbols to separate numbers depending on the language and region. So, in some countries they use a comma instead of a decimal point to separate the whole number and the decimal; therefore, if you have the same number as before, it would look like this: 3,14
Imagine you are reading your favorite book, and you want to read page 3 to page 4, having a decimal separator is like the page break. That little dot "break" the number in half so we can pay attention to each part along with the rest of the number.