I use Elo/MMR interchangeably here, they mean the same thing.
You have two MMR's, one for wins and one for skill. Skill is calculated on a ton of factors, based on ability usage, if you should be winning a 1v1, if your smoke helped take a bomb site, entry fragging, etc. Your win MMR is more straight forward, win games it goes up.
Lower ranks your skill MMR matters more, if you perform better it will up your Skill MMR which will make your wins reward more Ranked Rating. As you get into higher ranks your Win MMR will start to take over. Until Immortal+ where it is exclusively Win MMR.
So, in short, everything is important. I would focus less on what matters and focus on getting consistent at the game. Being consistent, able to play well on a couple of agents and all maps, will help you climb the fastest. Good luck!