Can Someone Explain What a Rank Reset Is?

The reset algorithm works as follows (previous season mmr+1200)/2. This is what is called a soft reset and differs from the "hard" resets of seasons 1 and 2 in that every player was returned to exactly 1200 mmr under that system. The effect of the current reset system is that your mmr is regressed toward its initial starting value. Players with bronze mmr will start a new season closer to silver 5 mmr and players with diamond mmr will start the season closer to high gold low plat mmr. You will have to play placements again to be seeded. Unfortunately the bonus MMR gains only happen in your first 50 lifetime ranked matches so you won't receive bonus mmr and lip gains from these placements like in your original placements (unless you haven't yet played 50 lifetime ranked matches of course. The value of the bonus mmr decreases from the first of these 50 to the last, diminishing returns. This makes climbing on "fresh" 30s far faster than on older accounts).

it means everyone has to do placements again. But it takes into consideration your last seasons ranked so the reset isn't "everyone is on same ground as if you just started" but more its a "soft" reset" so everyone is "unranked" but there ranked mmr got lowered a bit.