this means if you want to play full ad rammus, or full tank karthus, or full ad malzahar, or full ad katarina, or adc ahri, or ap ash, or ap zed (lich bane works!) or anything else off-meta, but your team doesnt agree with you, then you cannot play it! even if only one teamate disagrees then you cannot play it. if you play it and he reports you it is a valid report and u can get punished.
you can only play off-meta builds if your whole team agrees or if you go premade with 4 friends who agree on that.
I myself played many times tank karthus mid lane going full tank items. and it went extremely well. sometimes i even carried the games single handedly. but as u can imagine, if u get in lobby and u say i will play karthus tank noone will agree on that.
anyway... just wanted to share it with you. so now you know if some1 goes off-meta build, you can validly report him after the game :D
EDIT : PROOF :
(the rioter answered me more questions. one important one was that if you call an intentionally feeder as "troller" or "flamer" you can get punished, even if that guy is indeed intentionally feeding/flaming.)