I must have missed this, but I am not sure what += actually does.
never mind, I just needed to continue to the next section which explains everything.
It is a shorthand notation for
counter = counter + 1
If you want to type less, you can do
counter += 1
Which does the same thing: adds 1 to the current value of counter and then replaces the value of the variable counter with that new, incremented value.