Any hand with an ace worth 11 can be considered ‘soft’. So, if you had a six and ace, you would have a ‘soft 17’.
This matters, as it helps inform your next move. With a normal hand, you may be tempted to stand on 17 – but with a ‘soft 17’ it’s recommended to hit. This is because aces can be high or low, so you have more chance of staying within the 21 limit than going bust.