It's an AIRMET, just like the ones you learned about in private pilot training aviation Aviation Weather 101. Not sure where you heard that, but Foreflight does not have a nationwide meteorological staff forecasting weather, sitting at airports watching, or creating weather briefing content. The information comes from the national weather center. Go to the graphical G-AIMET online, and you'll see the same. information
From the FAA's Aviation Weather Handbook:.
"An AIRMET is a concise description of the occurrence or expected occurrence of specified en route weather phenomena which may affect the safety of aircraft operations. AIRMETs are intended to inform all pilots, especially VFR pilots and operators of sensitive aircraft, of potentially hazardous weather phenomena. AIRMETs contain details about IFR, extensive mountain obscuration, moderate turbulence, strong surface winds, moderate icing, and freezing levels"
It means meteorological conditions are conducive to the formation of the subject weather conditions,. In the case of Sierra it depicts areas where IFR ceilings either are or expected, i.e.. a forecast). Like all forecasts, it is informational and might not actually occur. Those who get a briefing compare that to other things, such as METAR for current conditions and TAF for an update on expected future conditions.