The Triquetra comes from the Celtic tradition, and has the meaning of infinite life cycle. It was a triple feminine symbol, very sacred to the Druids, who asked Catholic recycled to symbolize the Trinity and thus obtain the dual purpose of possession of a symbol, and shake off the previous meaning.
Precisely for this reason its origins are uncertain, and significance, other than to generically represent the infinity of life cycles, went almost completely lost. In Ireland, in fact, the country where the symbol was most widely used, the conversion of the population began before the year 1000 AD, and the old pagans were long opposed and fought. Of Triquetra we have also been found in Sweden, engraved on some rocks, and it seems to date back to the twelfth century.
Today Triqueta is only one Christian symbol then, and represents the trinity, although some groups belonging to neo-pagan religions, are trying to get back to symbolize the mother earth and its connections, or a trinity of sky, sea and earth.
But in the world of tattoos it is difficult to see it associated with these meanings, and often is only chosen for its stylistic form and vaguely tribal, certainly very aesthetic and graceful.