What is the difference between that and
Free Dining?
Free Dining = pay rack rate for your room and park tickets, get the dining plan for free (Quick Service if you stay in a Value Resort, Standard plan if you stay in a Moderate or Deluxe).
Stay, Play & Dine = a package offered the last 2 years or so in the winter. Last time, it was Pay rack rate for your room, get your tickets and dining plan at the child rate for adults. Disney hasn't flat out said that this year, but people on here have been reporting that using that as a base has come to a pretty close estimate.
Also, if you are staying at a Value resort but want the Standard dining plan, you pay the difference between the Adults Quick Service and Standard Plans in ADDITION to the discounted S/P/D rate. If you do the math for that, the child price plus the adult different makes the dining plan about $35ish/day for an adult.
You need to do the math and see if you'll save more with a room-only discount or with the S/P/D rate. For me, with 4 adults in 2 value rooms, we'll do better with the S/P/D discount since we were planning on a sit down meal every day anyway.