Map packs are groups of 3 community levels bunched together in a single bundle. Upon completion, you are awarded with a secret coin, so it will also help you progress through the secret coin set of achievements.
Currently, the easiest map pack to complete is the alpha pack. All of the levels inside of it are rated as easy difficulty. Keep in mind that despite the levels themselves being easy, they are really flashy, which can make the levels more difficult than they should be.
When you have completed a map pack, in order for it to count for this achievement and for you to get a secret coin, you need to go back to the map pack selection screen, and click on the secret coin where the "View" button normally is.
If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions. If you notice any typos or poorly phrased sentences, please let me know.