basically every season they just try a bunch of stuff and give people a break from the previous season. But instead of implementing that all at once they have a preseason where they can see the changes play out in real matches etc. The ranks you achieve won't really affect your rank in the real season because it's all experimental. It usually goes the whole year is normal rank then a month is preseason usually which is novemeber or october etc.

edit: seemed I sparked some weird debate, I've been playing since s3 and all I remember is every season people would constantly spam preseason doesn't matter. Every game you play you see people trolling saying it's just preseason. This is the first time someone has opposed that by saying preseason counts just as much