[Question] Meta. What is it?

No, seriously. I hear the word thrown around all the time in youtube videos, forums, reddit, everywhere. "The new meta" "off meta picks" "ingame meta". It doesn't mean anything. And I've looked around online to try to find a definition and to my surprise I came up short.

I feel very out of the loop here. Everybody seems to interject this word into their commentary without ever explaining or even hinting at what the fuck it means.

Can someone please enlighten me :)