What does "end of season" means ?

I see it more often being mentioned but, as a new player, I never saw an "end of season" happening. Should I save RP/IP for anything cool ? Are ranks deleted and everybody starts from scratch ? Are runes changed or new ones hit the shelves ? People dress in those cool green/brown Autumn colours ? New champs ? New abilities ? Nerfs/Buffs ? Everything stays the same and its business as usual ? And, when is it exactly the "end of season" ? So, what happens !?
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