Why is 121.5 called "guard"?
It is called guard because everybody is supposed to listen/guard the frequency just in case someone has a problem.  

Guard definitions:


  
  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.