What is a "guarded" frequency and how does it differ from a frequency that's unguarded?
It is called guard because everybody is supposed to listen/guard the frequency just in case someone has a problem.
The AIM 6-3-1(h)(1) says (emphasis mine):
121.5 MHz and 243.0 MHz. Both have a range generally limited to line of sight. 121.5 MHz is guarded by direction finding stations and some military and civil aircraft. 243.0 MHz is guarded by military aircraft. Both 121.5 MHz and 243.0 MHz are guarded by military towers, most civil towers, FSSs, and radar facilities. Normally ARTCC emergency frequency capability does not extend to radar coverage limits. If an ARTCC does not respond when called on 121.5 MHz or 243.0 MHz, call the nearest tower or FSS.
I wonder whether it comes from the French, where the verb "guarder" means "to keep" or "to reserve" or "to keep in reserve". A "guarded" frequency would then mean "a reserved frequency" which makes perfect sense.
First, and most relevant today, all facilities are supposed to "guard" (monitor) the frequency if able. Most ATC facilities and Flight Service stations monitor 121.5MHz, as do many airliners or aircraft with two radios. Lnafziger already gave you the relevant AIM paragraphs that talk about this.
Second, "guard frequencies" in general radio terminology are frequencies that have extra protection in the band.
121.5MHz in the USA is guarded in both respects - monitored as Lnafziger describes, and afforded extra band protection (50KHz on either side, 121.425 - 121.475 MHz & 121.525 - 121.575 are unusable per AC 90-50).