I see a lot of debate over what "meta" means, and I'd like to throw my own hat in the ring.
I would argue that a playing a game at the base level (i.e. playing without meta) consists of (a) finding different strategies to use, (b) figuring when is appropriate to use each strategy, and (c) executing strategies optimally. When the game is first being played, most strategies haven't been discovered. At this stage strategic play consists of (without loss of generality) player A using a strategy they think is effective, player B devising and using a strategy that will be effective against the strategy used by player A, Player A adapting in response, and so on. This is strategic play, but it isn't a meta-game. The meta-game arrises when players A and B are both experienced enough at the game that they can debate which strategy is the objective best. The meta then becomes the agreed upon dominant strategy (or set of strategies). In the base game (i.e before the meta develops) strategies exist mostly independent of each other, while the meta-game consists of fitting strategies into a framework.
The meta can change because of external or internal forces. An external force is a change to the base game, which is common to e-sports, and less common in actual sports. In e-sports, most games will tweak how the game is played (change the cooldown of abilities, change the size of a characters health pool, etc) every month or so. This will change which strategies are best, and therefore change the meta.
Internal changes arise from the changing skill level of players. As players get better at the game, hard to execute strategies will become more viable, while easy to execute strategies will remain at about the same level of viability.
This article shows a great example of what the pro Overwatch meta (the game I'm most familiar with) looks like and how it evolves, and it's (mostly) accessible to non-Overwatch players: https://overwatchleague.com/en-us/news/23053244/the-meta-rep... (WARNING: autoplay video).