Steve Silverwood ...

^ ... ALWAYS monitor the GUARD frequencies ...

Emergency frequencies are guarded, or watched, therefore they are often
spoken of as guard frequencies. "Guard" is a verb in this case, not a noun.
When units are transiting in company with each other only one unit need
maintain the guard, and that unit is referred to as having "the guard". So
when "guard" is used as a noun it refers to the unit maintaining the guard --
the unit watching the emergency frequencies. Through misunderstanding this
term has become somewhat corrupted and I often hear sheriff aviation units
calling 121.5 the guard channel.

Frank