Mastery Chests

http://s.lolstatic.com/site/2016-season-update/0da6eedea590698dd1e4a66dee1029f3b50539a5/img/champion-mastery-panel-1.jpg Mastery Chests Do I understand?

1. I play cho for 3 months.
2. I drop chests.
3. I open my chests.
4. I get skin for Lucan's guns. How does this relate to my favorite hero ?!!!!!!