MMR Brackets in Dota 2 are divided by 1k MMR separation which represents the skill of each player. The higher the MMR the better the player, but it dosent have to be right everytime.

So starting off with the lowest MMR bracket 0-1k MMR
Personally i have never never been there, but gathering info from the internet and some of my friends, man its really bad.
Players use to click ond their abilites and cast them afterwards, even not knowing what every item in the game does.
Overall these players are awful at the game and there are surely no teamfights in this bracket.

1000-2000 MMR
In this bracket the players are still bad, but way better than the first one. You shouldn´t be surprised if you find any decent players here, because they are stuck in this bracket from guys who lose their chill early in a game and then rage quit. Teamfight is still hard to find tho.

2000-3000 MMR
Here you can start finding some good teamwork and people that are pretty good at the game. It can be called the ,,Baby support bracket´´ because there are people that actually love to play support and buy the necessary things in order to get the win for the team. Seeing toxic people in this bracket is still easy.

3000-4000 MMR
As far as i know this is the bracket where the majority of the Dota 2 players are stuck. So to get out of this bracket is possibly the hardest thing to do in the game.Even though you are thinking ,,finally here i can play with some amazing players´´, nope, toxic players are maybe the most present thing in this bracket, because of the whole competition that is going on.

4000-5000 MMR
Well here the real Dota 2 starts. Less arguing about support or carry role and more teamwork. Coordination is the main thing if u want to win a game. Smoking and gangking is more present, players are more efficient at farming and supporting and most of the players know how to use their heros abilities properly. One more thing, be aware of account buyers.

5000-6000 MMR
Players here are improved a lot. Hardly finding any mistakes which leads to pure fighting skills and competition.Even here account buyers are still existing.

6000-7000 MMR
The number of people starts to decrease dramatically here. Because of the skill and coordination that is required to come here, from now on only strong players will appear. In this bracket you can even find professional Dota 2 players from which you can learn a lot.

7000-8000+ MMR
All of the Dota 2 professional players are here. No mistakes no nothing. In one word the God bracket in Dota 2.