The lawn at Blossom Music Center is a general admission seating area. Some tickets are labeled with a row and seat, but these details are irrelevant. All lawn tickets are the same. Ticketholders are encouraged to arrive early in order to secure a desirable spot on the lawn.
Don't forget your lawn chair and blanket - although for more popular shows (like the Def Leppard/REO Speedwagon concert you're attending), it is highly likely that other lawn patrons will be standing during much of the show. Another reason to arrive early - if you secure a spot at the front of the lawn, you decide whether to sit or stand.