The 8th rank of a character ability requires either Omega materials or Zeta materials. Omega materials are uncommon, but are given out occasionally as rewards for level 70+ activities. Omega abilities are usually very good. To get Zeta materials, you must progress far into the ship battle mode. Zeta abilities are almost without exception very good, and some of them are incredible.
"Meta" refers to meta-game, which is the style/form of play that is determined (usually by consensus of the players) to be optimal for any given time or place in a game, and so the crowd of players follow it because they all want to win. It is the information about "how to play and how to win" which you can learn by reading forums. "Meta" because the information comes from outside of the game.
In SWGOH, the meta for new-ish players is widely considered to be Rebels: Wedge + Biggs, probably teamed with either Lando & Stormtrooper Han, or Baze & Chirrut. When players gain access to a greater variety of Dark Side characters, the meta tends to move toward Empire or Sith teams because they are capable of beating Rebels in arena.