I'll build on what Tr4shB4NSYST3M said.
Part of being good at solo Q is understanding the strength of your champion and what job you need to do on your team to help win. Even playing the same champ, it's not enough to know what that champs strengths and weaknesses are; you need to know how those strengths and weaknesses interact with the win conditions for your team.
So, for example, I play a lot of Taric. Taric is good at teamfighting. Even in teamfights, though, there are many strategies you can use to fight. One strategy might be to use bastion on my marksman player and sit on them, making sure that they don't die to the best of my ability. Another strategy might be to use bastion on my frontliner, to give them a big shield and the armor to use it really well, so they can tank as much damage as possible.
Those two strategies then inform how and when I use my abilities. I can stun on the frontline if my W is on a tank, but then maybe I want to stand next to my markman anyway, just to threaten anyone who wants to dive them. How/when do I ult during all this?
See what I mean? There are so many ways to play it. And that's already assuming that we even want to teamfight; we might not, depending on the game state and our team!
So that's what people are usually referring to when they talk about win conditions. You need to think about what your team needs to do to win. In the games where I play poorly, I often am just trying to get a stun on someone every time I see the enemy team. Maybe I get a killer 3 man stun, and it looks really pretty, but if my team has no follow-up, it doesn't matter. Me engaging well may not help us win at all. So you want to be thinking about that, ideally every game you play.
. Walk around one-shotting everyone from stealth in .1secs. Because balanced counterplay 200 years of development experience.
