Odometer and chronometer: what are those?

These are optional fields for locomotives, so you can note how much a particular engine has run.

• Odometer — a mileage-style reading.
• Chronometer — a run-time / hours reading.

Both are free text (up to 100 characters), so you can record them however makes sense for you — "approx. 12 hrs," "scale 1,450 mi," or just a date you last logged it. They only appear for locomotive-type pieces.

They're handy for tracking maintenance intervals on engines you actually run.

See What can I record about a piece? for everything else you can log.