Hi chach. It refers to how far forward the cartridge is mounted in the headshell.
Strictly, it means the exact distance past the spindle that the stylus sits, when on a straight line that passes through the stylus, spindle and pivot.
BUT it has all got a bit mixed up, and sometimes now refers to other dimensions within the headshell that relate to mounting position. So the answer depends on the context, I'm afraid. But it always relates to catridge position within the headshell.