To return partner to his original suit at the same time making a jump bid. Jump preference is encouraging and shows at least three-card support for the suit bid (exactly three-card support if it is a major).
For example:
If partner opens 1♥ and rebids 2♣ over your 1♠ response, you give jump preference to 3♥. If partner opens 1♦ and rebids 2♣ over your 1♠ response, again you give jump preference, this time to 3♦.