A decimal number is a different way of representing numbers smaller than one. You put them after a full stop (the decimal point), for instance 0.5. The first digit after the decimal point represents tenths. If you sliced a cake into 10 slices, each slice would be a tenth of the cake. So 0.5 is the same as saying 5 tenths, and can be written . This is the same as because can be reduced to .
Further digits after the decimal point represent hundredths, then thousandths, and so on. For example the number 2.25 represents 2 whole numbers, 2 tenths and 5 hundredths. But it’s easier if you think of the digits after the decimal point as being a single fraction. Work out what the last digit after the decimal point represents. For example in 2.25, the 5 is in the hundredths position. Then take all the digits after the decimal point and put them over whatever the last digit represents. So .25 becomes 25 hundredths, or . 2.25 as whole numbers and a fraction would be .
Sometimes numbers need a lot of digits after the decimal point to show the exact number. In most cases, however, it is sufficient to show two decimal places. For example you would usually represent as 0.33, even though the number actually has a lot more digits after the decimal point.