That's the Altimeter screen. The black bar shows you the latest changes in altitude. If you press "Adjust" button, it will show you how many meters (or whatever measurement unit you use) you've climbed or descended. For example, if you climb a mountain it will show you in the graph the "ramp" that you've climbed.

The top number is the current time. The bottom number is the altitude compared with the sea level.

You can calibrate these values, but I forgot exactly how.