The metagame (also called the meta) is the current trend that is often considered to be the optimal way to play the game by the community. This always changes over time, and is driven by community opinion, professional players, and balance patches.
In simple terms, metagame in competitive Dota 2 is a set of developed habits during the picking phase of the game. It usually stems from one or two heroes being considered very strong or even over-powered after the most recent patch.
For example, Lich gaining XP for his Sacrifice as well as significant buffs to his Ice Armor in 6.79 resulted in him being picked and banned a lot in the competitive scene. At first, he seemed to be overpowered, however the answers to the dual or solo-mid Lich, offlane Lich and Support Lich were soon discovered. That in turn, has provoked a response to a response to Lich. And so on and so forth.
The cycle of hero rotation in the competitive scene without a single point of convergence is called a "healthy" meta. In all these years it did not converge on a static best first-pick lineup against best-response, second-pick lineup.
Competitive Meta, as well as a variety of several other factors contribute to the Public Meta.
The Public Metagame affects the vast majority of players. The current popularity of heroes can give you a pretty good idea of what you are more likely to face in any game.
Looking at the professional scene can often give some insights on what is going to be popular in the upcoming week or two. At the higher levels of play people try to copy the professionals a lot and trust their judgement in one way or another.
An additional point to be made is that some of the heroes are not available in Captains Mode after patches/reworks. Moreover, new heroes usually do not make an immediate transition to the professional scene. They will be Dark Horses for the players for a day or two and can range from interesting, but not very useful to borderline overpowered.
Better players will figure out how to react to them in one way or another. For some of them the Patch Notes themselves will be enough. Others will need a game or two with or against the hero, to understand the strengths and weaknesses.
- Overpowered - A hero with too many strengths and not enough weaknesses.
- Underpowered - A hero with too many weaknesses and not enough strengths.
- Dark Horse - An undiscovered hero found to be a strong choice well after the meta for the current patch was already established. Can often cause a shift in the meta, continuing the cycle of hero choices rotating in response to other choices.