Is Required Of Activity2

Hi Guys,

Can any one help out to me for  to understand the use of Is Required Of Activity2 check box ?

If I checked Is Required box Of Activity2 then what happened ?

if I do not check Is Required box Of Activity2 then what happened ?

Because I tried to both option checked or not checked  and complete the activity 2 of Project. I did not found any difference.

Thank in Advance.