What "S-tier" means in VGC
S-tier Pokémon are those that opposing teams must specifically plan to answer. Flutter Mane is in this category because it OHKOs nearly every non-Steel Pokémon in the format with Specs/Booster Moonblast. Iron Hands is here because of its absurd bulk + Drain Punch healing + Wild Charge spread. If your team has no answer to either, you'll lose 40%+ of your matches.
A-tier vs. B-tier distinction
A-tier Pokémon appear in 15%+ of teams at top-cut Regional tournaments. B-tier Pokémon appear in 5-15%. The line moves between Regulations as rules change.
Current Regulation H
Regulation H bans all Paradox Pokémon, Treasures of Ruin, all Legendary/Mythical Pokémon, and Pecharunt. The metagame is much more "normal" Pokémon focused — Talonflame, Annihilape, Tinkaton, Ogerpon dominate where Flutter Mane and Iron Hands previously locked the format.
For builds of any of these Pokémon, see our competitive builds hub.