As other answered it will be a soft-reset.
But nobody touched what that meant...
Your elo will drop significantly the higher elo you have.
And your elo will rise significantly if you happen to have very low elo.
What is low and high? That is for you to decide, the "ground zero" is 1200, and you can use various of mmr/elo checkers out there to see ur elo.
High diamond/masters are around 2400-2500 elo
low diamond 1900-2000/plat 1600~
gold 1400-1500
all the numbers are approx.
So lets get back to what a soft reset it.
Your elo wont go back to 1200. It will be directed TOWARDS 1200.
So everyones elo will get closer to 1200.
As far as i know this is simply done by adding 1200 to your elo, and then dividing this sum by 2.
So say your elo was 1600 this season, after the reset it will be: 1600+1200/2=1400
So now u may ask, huh, what will my division/tier be? this is not that relevant, in the end elo/mmr will always determine where u will be placed; if you get placed at silver 5 and u have the mmr of an avarage gold 5 player then it means that u will play vs gold's gaining alot of lp, losing less lp, to reach the division/tier in which ur mmr/elo belongs to. But if you happen to lose alot of games during this "grind" your elo will get closer and closer to that of ur current division.
Also, to make the ranked system "fair" and not have too many masters players around plat elo, everyone receives a small "elo""mmr" boost the first 30-40 games of ranked play.
Normally every game ull lose and win 12-13 ELO (while ur lp gains loss may differ elo is usually always static (unless u game the system)) During these first 30-40 games u will gain/lose 50-40-30 elo per game diminishing, getting less and less till u hit roughly 60-70 games. (people from season 2 will remember this)
tl;dr elo rules all.