Probe characteristics- what does CAT 1 and CAT 2 mean?
I believe you are referring to Measurement Category

The 300 VRMS refers to the largest voltage the probes are rated for use with.

From Wikipedia (Credit to all those that contributed)
http://en.wikipedia.org/wiki/Measurement_category


  CAT I is applicable to instruments and equipment, which are not intended to be connected to the mains supply.
  
  CAT II defines circuits which are intended for direct connection into mains sockets or similar points. The energy in such installations should be limited to below 100 A continuously (or below 500 A for voltages not exceeding 150 V).
  
  CAT III is for circuits which can be connected to the mains installation of a building. Energy is limited by circuit breakers to less than 110 000 VA with the current not exceeding 11 000 A.
  
  CAT IV includes circuits which are connected directly to the source of power for a given building. There are very high levels of available energy (e.g. limited only by the power transformer) and arc flash can occur.