Overview shows your plan, how many bookings you have left in your quota, and how many active units you manage. From here:
Upgrade Plan when your booking quota runs low.
Switch Plan to move between paying per booking and paying per unit. Elev8 shows how much you could save based on your current volume.
You'll also see your remaining bookings at the bottom of the left menu, with Buy More Bookings. See Getting Started for how the plans work.