I THINK you mean the word decimal. It is a way of writing fractions that are in 10ths. For example, 1/10th would be 0.1 and 2/10ths would be 0.2 By adding another digit to the right of the decimal point, you can write hundredths, another digit thousandths, etc.
2.3 2 is two wholes, which go on the left side of the decimal, and 3/10ths is .3 because the first decimal place to the right is 10ths. Hope I helped!
0.3 is a already written as a decimal. It is the decimal version of 3/10ths.