I think they meant 2,10Ghz (2100Mhz) It's the frequency a component (like cpu, gpu etc.) is working on. More in depth info about Hertz can be found on the usual wiki pages : http://en.wikipedia.org/wiki/Hertz
1 HZ means one cycle a second, in the Processor world 1 hz means 1 calculation per second.
2.10Ghz or Gigahertz = 2.1 Billion operations per second that the processor can calculate!