Soft 17 is a term used in the casino game of Blackjack and other related games like Multiple Action Blackjack and Spanish 21.
In blackjack all the cards have a value associated with them. Face cards are worth 10, aces are worth 1 or 11, and all others are valued equal to their rank.
A "soft 17" hand is one that contains at least one ace and the total count of the cards can be either be 7 or 17. For example a hand containing an ace, 2 and a 4 is a soft 17. (A hand with an ace, 9, and 7 would be a hard 17 because the ace cannot be an 11 in this hand without making the hand bust.)
A soft 17 hand can always safely take another card (called a "hit") without fear of going over the total of 21 (a "bust"). Depending on the game played (and the current texture of the deck) you may or may not want to take another card in this situation.
The game you are playing should clearly state on the table layout what the dealer must do when confronted with a Soft 17. The dealer will either have to always hit or always stand depending on which casino you are playing.