Lets say we receive some value and it is null and we are going to use that in our code

var divident=null;
var value=divident / 2;

Above line will throw a null exception.
To avoid that one might add a condition that could be

if(divident !=null)
{
var value=divident / 2;
}

a better way could be like below:

var value= (divident ?? 0) / 2;