Once upon a time there were summoners, these summoners started devastating wars which destroyed great tracts of land.At the end of the last great rune war some countries and summoners came together to create the IoW which had the goal of stopping any conflict that would result in more rune wars.
The IoW created the fields of justice to settle political matters, where summoners and champions would work together to represent various city-states in a sort of gladiatorial combat.There were various story-lines involving corruption of Institute , Freljord's struggle to become a full fledged nation, etc. There was political intrigue, character pieces, and a very decent amount of goofy stories also. Blitzcrank had a dating service at one time(which found rammus' one true love was a cactus.)Some of these stories were released through an in-universe newspaper called the "journal of justice".
About 9 months ago,Riot announced that the game and lore would be separate entities, in the process making summoners, the Institute of war and many other things non-canon now, meaning they were deleted from the world as if they were never there.
At the moment some things that are likely not to change drastically are characters associated with the freljord
, characters associated with the shurima event
and characters associated with noxus
.
TL;DR There were summoners they stopped world war 3 through gladiators, Rito removed them and now everything is in a grey limbo of uncertainty.
Its kind of a weird hour, so let me know if my explanation was satisfactory. I'm sure you might have some follow-up questions.