The Celtic symbol for trinity has a myriad of symbolic meaning.
We see the trinity motif in Celtic knots, as well as in symbol-form like the triquetra and triskelion (a.ka. triskele or fylfot)
To the ancient Celtic mind, it may also signify the lunar or solar phases. This conclusion is made as we see the trinity/triquetra motif alongside other solar and lunar symbols in ancient remants and archeological digs.