What does this point mean?

This is a sign for the center of mass.

The load diagram for your robot type will give you more detailed information about it. I would highly recommend to take a look on it as it is one of the most important things to consider.

Quote from leoo

SO...the center of tool should't over the range?

Should. It depends on the payload. That picture shows us workspace only... the range of faceplate

IIRC, that represents the Center of Gravity/Mass used for the official Max Payload that the robot is rated for, and is used when the robot is factory calibrated.

So, at KUKA, there is a L-shaped bracket that holds a set of steel disks, for every robot model. So for a KR5, there is a 5kg weight whose L-Bracket holds it at that particular distance from the A6 mounting flange. In your diagram, that would be held at position of X 100, Y 0, Z 80 from the center of the A6 mounting flange.

Payloads of smaller mass can have their CGs further from the flange. It's a complex relationship. KUKA offers for download a spreadsheet where you can insert you robot model, the mass and CG of your payload, and the spreadsheet will generate a report detailing where that Mass/CG combination falls in the robot's inertial envelope.

There are also differences between static and dynamic loading. A payload whose inertia is within the robot's carrying capacity may still force the robot to move/accelerate slowly.