It means all your graphics have to be in dimensions divisible by 2, or you have to change how you lay things out if not (and do more work to scale odd dimensions). Even when everything is, graphics can look terrible when you resize naively, depending. Sometimes you have to recreate graphics manually at different dimensions to get better antialiasing and things.
The interesting thing you'll find, as you continue to grow your capabilities, is that you can eek out more performance where you thought there wasn't any in the past. Keep up the learning and the great work.