I heard a pilot said they are located at big airports like LAX. But I couldn't find any information about it.
A warm up apron is a portion of the airfield close to the runway entry, where the engine can be warmed up before taking off. It seems to be more frequently used in military documentation.
There are such aprons at Cairns Army Airfield:
Found in FAA regulations, e.g. AC 150/5340-30:
PAPI LHAs must not be located closer than 50 ft from a crossing runway, taxiway, or warm-up apron or within the ILS critical area.
But also in other not regulatory documents, here for synonymous of hardstand:
... construction of the RMP test section and warm-up apron (hardstand) at the east end of Runway 04-22. The RMP warm-up apron was successfully constructed and has been opened for traffic since October 1992.
A vintage post-card:
These are sometimes called run-up aprons and its an area where a pilot can perform final preflight checks before taking off as well as engine run-ups and tests where a pilot can do these without worrying about jet blast or propwash kicking up FOD all over a parking apron. At military bases, this is used as a final once over by ground crews as well as ordies removing the safeties of all external ordnance thus arming the weapons prior to takeoff.