It almost depends on who you ask, because it holds meaning to both Christians and non-Christians, but their definitions differ based on personal beliefs. Christians believe that the three points represent the three elements of the trinity: the Father, the Son, and the Holy Spirit (or Holy Ghost).
Pagans, which are basically modern offshoots of the original Celts/Druids, take a more tellurium approach to the symbolism, seeing it as a representation of the natural forces: earth, air, and water. On a more spiritual level, it has also been known to symbolize life, death, and rebirth. There are also several other meanings that basically boil down to one thing: three separate entities that are inter-connected.