What exactly does the iPhone accelerometer measure?

The apple documentation for UIAcceleration class says,

"When a device is laying still with its back on a horizontal surface, each acceleration event has approximately the following values:
x: 0
y: 0
z: -1
"

Now, I am confused! How can the acceleration be non-zero, when you clearly say the "device is laying still"?

UPDATE

Judging by the responses, I think this should be called something like 'forceometer' or 'gravitometer' and not accelerometer!