A safe room, as defined by the Federal Emergency Management Agency (FEMA), is a room - preferably below ground - in which people can take shelter from a tornado.
If such a room is below ground, it may not be the safest choice if told to stay at home and shelter-in-place during a weapons of mass destruction event due to the possibility that some contaminants may seep into rooms below ground level. For this reason, the Red Cross recommends and endorses having a safe room in areas where tornadoes are a threat.
However, do not confuse a safe room used for protection from windstorms with a room selected for shelter-in-place. They are technically different, although they serve a similar purpose. If a safe room for windstorms is above ground level and has no windows, it can also be an ideal location in which to shelter-in-place.