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.
- a state of caution, vigilance, or preparedness against adverse circumstances
- watch over in order to protect or control.
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.
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.
This is done for two reasons: to prevent use of the guard channel from interfering with other channels, and/or to ensure that the guard channel is always clear and that adjacent channels won't "bleed over" and render it unusable. This mattered more in the days of tube radios with poor frequency alignment & confinement - modern radios bleed much less, but some frequencies are still afforded extra band protection.
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).
From my days as a Navy communications officer, if you guarded a frequency such as 121.5MHZ you also had a transmitter set for that frequency as well as a receiver. If you only monitored a frequency, you only had a receiver tuned to it. We at least monitored both 121.5 and 243.0 any time we put out to sea.