Indicates that an ace is involved: A 7 and any card worth 10 would be a "hard" 17, and a Ace and a 6 would be a "soft" 17. It is considered soft due to the fact that the Ace can be counted as a 1 or an 11. A "soft" hand is a hand you can hit and get a 10 and still have the same hand; an Ace is ALWAYS involved.
To answer your question, if the dealer must hit a soft 17 this means that if the dealer has an Ace, 6 hand they must take another card and can not stand on the 17.
It is important to note that Dealer's Hitting of the "Soft 17" is very advantageous to the house. Such games should be avoided where that is an option.