what does this sentence mean?

Difficult to explain this without a sketch...but you've probably got a diagram.

It's describing the centre of gravity of a load, and its effect (upsetting moment, overturning moment) on the stability of the lifting machine (a forklift or other hydraulic device, digger, loading shovel, etc.)

The centre of gravity is measured horizontally from the backstop of the forks (or bucket) and vertically from the forks themselves (or the bottom of the bucket).

Once you know the weight of the load, and the turning moment, you can (for example) draw up a table of maximum loads at different reach (radius) for a given machine.

Of course, in a forklift, the reach (radius) is fixed.

EDIT: or not quite.
The guides for the forks (or lifting platform) often- but not always- slope back, so that as the load is raised, its centre of gravity moves closer to the centre of the vehicle.

Was any of this helpful?
Have you solved your problem yet?

I don't know if it helped him, but it certainly helped me.

Thanks for your patience and time...O(∩_∩)O~

So was it helpful? Are you any further forward? That piece would not lend itself to direct translation, and that is why I replied as I did.

It was very helpful. Sorry I did not see your post.
You are very kind to help.