The tournament meta shows the decks (and their variants) that are the most successful (or the most present) in a type of tournament over a period of time.
"All decks" will show every single deck that was registered in the selected tournaments for that period. "Best decks" will only select the decks that went 3-0 in Fight Night and Singles Night, or X-2 in other tournaments.
A deck is considered a "variant" of another deck (rather than a different deck) if over 75% of the cards in the main deck (excluding resources) are the same. Reserves, resources and gems are not considered.