wait wha-----

Please, someone, explain to me what a series is. I'm so confused. It says I'm in a series and suddenly I join a custom game for fun and it says I'm Gold 5 from bronze 4 lol. It still says I'm bronze on my profile but not when I join a game. Does it have something to do with a series? And also, wtf is a series in the first place

Edit: Doesnt say I am gold anymore. Prob was a glitch but wats a series