What is the most important part of the decimal system

The decimal... lol  thats what mt grandmom told me

The decimal point without the decimal point there is no decimal at all. 
i hoped that works ill give you an example if you are still confused 

if you are trying to turn this number 89 into a decimal you would have to do 8.9 if you don't than  89 is just a number not a decimal if you forget the decimal you will more than likely get the question incorrect.