An empty leg is a one-way repositioning flight: the aircraft — a Bombardier Challenger 850 — has to fly Denver to Des Moines anyway, so the whole cabin is offered as a single one-way charter at the operator's repositioning rate rather than a round-trip one. You charter the aircraft, not a seat, and the date and route are fixed by the operator's own schedule. This leg covers 588 miles, about 1h 29m in the air. The cabin seats 15.
Empty-leg inventory turns over continuously — a leg is live until the operator sells or cancels it. If this one is gone by the time you call, the links below show what is live on the same route and at the same airports right now.