Soft vs hard
Ace+6 is soft 17. If you hit and get a 10, the Ace becomes 1 and you have hard 17. Soft hands are flexible — you can often double or hit safely.
Key soft plays
Soft 19–21: stand (sometimes soft 18 doubles vs weak dealers). Soft 18: double vs 3–6 if allowed, else stand vs 2/7/8 and hit vs 9–A. Soft 17 and below: often hit; double vs certain weak upcards when the chart says so.
That single play trips up a lot of casual players.
Tip: Do not treat soft 18 like hard 18. Soft 18 vs a 9, 10, or Ace is usually a hit.