Hi Govind
Naresh answer is right but i want to put more clarity there.
Well first of all if you want to create you own variant in fb50 it is very simple. First put you mouse n the column and trag it where you want, and those columns you do not want to see hide them and then click on far right configuration button in the line item section as Naresh has explained there. and the just creat your variant buy give it a name.
SHDO is used for many purpose but the main purpose to restrict user from some feature of with in a screen or some functionality as well. For example some you do not some of the user to see the park the document for this purpose you can create a fb50 variant to hide the park push button.
i hope it is explained