Question : How to use "IsNull" with varchar datatype?

can anybody please explain what the second line of code is doing in the following statements.
I am trying to use some existing functionality, but I am not getting how to interpret this.

@GMTOffset   varchar(5)      
if len(ltrim(isNull(@GMTOffset,''))) = 0        set @GMTOffset  =  null

Thanks in advance!

Answer : How to use "IsNull" with varchar datatype?

Sure...it trims any spaces from the left and right of the variable and then looks at the length of the value in the variable.  The ISNULL function puts an emptry string in place of a null valued variable.