You have opened with a two-suiter (you have at least 5 and 4 cards in your two longest suits), your partner has bid a suit and you don't have a fit. You want to show your hand.
If your second suit is ranked lower than the first one, bidding your second suit is not forcing and shows 12-18 HCP. If you have more, you need to jump.
If your second suit is ranked higher than the first one, bidding your second suit is called a reverse. It's forcing and shows at least 17 points.