A long-lead item is anything with a delivery time long enough that it has to be ordered well ahead of when it's installed — electrical switchgear, elevators, custom air handlers, structural steel. Lead times can run many weeks to many months, and they often can't wait for the normal submittal-and-award pace.
Managing long-leads is a defining PM skill: you track them separately, expedite the submittals that unlock fabrication, and sometimes release an order before the full commitment is signed to protect the schedule. Miss a long-lead and no amount of field hustle can recover it — the material simply isn't there.