Depends on the PCB and manufacturer ! - If I had to quess i would think possibly Jumper 1 - (usually used on a set of jumper pins on the MOBO to denote which jumper block is which (ie. J1 is the first set J2 the second etc.) - but to be certain would need to know what MOBO so you could look in the manual.
Back in the good old days (ancient history) the J1 jumper was a set of three small pins at the bottom front of a motherboard. There was a tiny plastic block that could be used to short two of the three pins. Shorting pins 1 and 2 was for normal operation. Shorting pins 2 and 3 would clear the CMOS and reset the system setup configuration to default settings. We usually used the J1 jumper to restore default settings because we experimented with settings in BIOS and things didn't exactly go according to plan.
Modern motherboards now come with a simple pushbutton to clear the CMOS.
There is a remote possibility the J1 jumper was used for something else but it is very very rare. Consult your motherboard manual to find out.