וּקְשַׁרְתָּ֥ם לְא֖וֹת עַל־יָדֶ֑ךָ וְהָי֥וּ לְטֹטָפֹ֖ת בֵּ֥ין עֵינֶֽיךָ׃
And thou shalt bind them for a sign upon thy hand, and they shall be for frontlets between thine eyes.
Read kabbalistic (mystical) interpretation of Deuteronomy 6:8: and classical mystical sources.