Probably means "First Pilot" military time, i.e. in some aircraft, you are left-seat qualified prior to becoming aircraft commander qualified. Two first pilots can take an aircraft for certain missions (primarily local training) without either being an actual aircraft commander.
In some aircraft, you are also a "First Pilot" if you are aircraft commander qualified, but not fully qualified in the aircraft mission yet, i.e. a C-130 pilot who can fly passengers around but isn't yet airdrop qualified.
Rule of thumb: if you signed for the jet, it goes in the FP/IP/PIC column.