Open time trips are trips that haven't been assigned to anyone. When lines are created by the planners (or PBS in our case) there are generally some trips that can't be covered and those trips go into "open time" where they can be picked up by pilots for extra pay. Eventually, if the trips don't get picked up, they are assigned to reserves.
At SKYW, if you request to pick up a trip from open time within 10 days of the trip they either have to give it to you at 150% pay or immediately assign it to a reserve line holder.