Welcome to the RadioReference HF Gateway. "HF" is an abbreviation for "High Frequency." See Spectrum.
High frequency communications occur in the 2-30 MHz band. Signals in this band can be refracted by the ionosphere allowing worldwide propagation. The distance varies depending on time of day, the solar cycle and current solar and geomagnetic activity, but in general it's best to listen below 10 mhz at night, above this during the day. Our HF Propagation article has links that describe these phenomenon in greater detail.
Many of the FAQs linked here are courtesy of DXing.Com from Universal Radio of Reynoldsburg Oh. and the Monitoring Times website.
The RR forum for this topic can be found here