Ok it might seems a bit confusing at first but let me try to make it easy for you or at least I could try.
At first we are declaring variable name which is var a; now we have a variable with a name a but we yet have to assign a value to it. So in the next step as you can see a = 7 now var a has a value inside which is number 7.
If we look at var b we have only declare the variable name but never assigned any value to it. So at this stage we have total of two variables var a which has value 7 and var b which is without any value.
In the task they said * Assign the contents of a to variable b .* which simpily means just make the variable of b to same as a. So what we could do here? here is the hint -----> = this EQUAL sign can will help you out.