In SPSS, you can do pretty much any computation using your variables to create new variables (or to change the values of your existing variables).
Let's say VarA has a value of 3, 2, and 5 for persons A, B, and C, respectively.
When you use the command: VarB = VarA*2
The values for VarB becomes 6,4, and 10 for persons A, B, and C. So you simply multiply every value for that variable (values for each case) by 2.
So the +1 in your syntax simply adds 1 to each case. If your original variable with 4 categories had 1, 2, 3, and 4 as the numerical values for your categories; your new variable becomes 2, 3, 4, and 5. It changes the numerical values for each variable.
Of course you can only use such calculations with numerical data.
I hope this helps.