it doesn't matter online so much but if you're going to an IRL tournament the meta is just an understanding of what's going to be played.

It can work on several levels: in general terms, the current meta has a strong evolutions presence and you have to have a deck that can cover Garbodor. Decidueye dominated the sun and moon meta to the point where again, you either had to beat it or play it.

Then you'll get more local or tournament specific meta. So if you play in a shop where there are 8-12 regulars who tend to use the same decks, that would be a very local meta and you could notionally tech against these decks.

In bigger events players need to make meta calls. So in Toronto there was apparently a lot of talk of Night March being likely, so for that particular event people played Giratina/Toad, which doubles with Chaos Wheel to stop DCE and quaking punch to stop items. That's a good example of a meta call, decks being used because of what expectations there are for other decks.

Most players will consider this when factoring what to play.

What decks am I likely to face? What decks am I happy using? How does this work together?

So if you know Drampa/Garb is going to be everywhere, what do you do about this? With Zoroark on the rise you might want to play a fighting type, but we also know that Tapu Bulu is big and will autowin against fighting, so where do you go from there? Metagross? Good deck, but loses to Volcanion. And so on. I think this is what led a few players back to Darkrai this weekend, they weren't sure about where to go and figured that Darkrai had decent matchups against most decks.

Greninja Break fits into the current meta because of field blower. It was a powerful deck, then people stopped it with Garbotoxin, because at the time there was no tool removal. Now we have field blower and Garbotoxin is only a temporary stall, so Greninja is viable again. It's a fine deck but only works if grass isn't overpowering. And at the moment that's fine, but sooner or later someone will make Lurantis great, Golisopod GX looks interesting, and either of those could just steamroller Greninja.

So meta means lots of things and I've probably over analysed it all, but you need to have a sense of where all the cards are in the overall meta to win games. It's what keeps the game fresh and challenging.