My VX2100s do not have an hours meter, so I'm not very familiar with them, so don't make any heavy bets based on this info, but I think that threading counts the number of hours a tape has been run through - played, recorded & rewound; operation is the number of hours the unit has been on; drum hours - I'm not sure but I think this includes played tape, paused and recording and "tape" includes paused time. I've heard that 1,000hrs is the magic number for drum hours. After that, you can expect trouble or to replace heads.
OK, I just "cheated" - here it is from another board -
HOURS METER
Operation: 30x10
How long the camcorder has been turned on regardless of mode.
Drum Run: 2x10
How long the head drum has been running, either in record, record-pause or playback modes.
Tape Run: 1x10
A shorter time than Drum Run, as record-pause isn't be counted.
Threading: 9x10
How many times a tape has been loaded or threaded into the recorder.