What is mastering a champ? Is it getting it to rank 5? Is it just being good every game. How do you master a champ? I want to know, because I want to master Evelynn
jungle, and Ekko 
mid. If anyone knows how to get good at tank Eve and glass cannon ekko, please tell me. Thanks!
Mastery 5 essentially means you've won as/played a certain champion several times. For jungle, I recommend you check out https://www.youtube.com/user/foxdropLoL (upbeat guy that you can learn a bit from)
For mid I suggest you check out https://www.youtube.com/user/lastshadow9 (He's an asshole, but this asshole knows what he's talking about. You'll learn a shit ton from him)
Streamers you should check out:
Foxdrop.
Phylol is great too.
Stonewall008 is perfect for learning the jungle.
Anyway to answer your question Lvl 5 mastery does not mean you have mastered the champ. It can be achieved simply by grinding. But it does visually represent that you understand the mechanics of that champion. Actually mastering a champion requires time and dedication that varies from champ to champ. Some champions have higher skill thresholds than others
. Some have lower ones 
.
I've gotten Vayne to rank 5 mastery, but I'm nowhere near mastering Vayne.
If you've mastered a champion you know every build for that champion, you know how much damage your abilities can do with different items on them, you know every lane matchup, etc. Basically you can use the champion to their full potential.
