It is a most ancient custom to include five knots in tzitzith. The following reasons are given:
1) The five books of Moses.
2) The five senses, to remind us to attach them to G-d.
3) The five words 'Shema Yisrael Hashem Elokenu Hashem Echad.'
The letters of the word tzitzith add up to 600, and if you add five knots and eight strings, it makes 613, the number of mitzvos.
- adapted from Tzitzith by Rabbi Aryeh Kaplan