John Gill Commentary Deuteronomy 6:8

"And thou shalt bind them for a sign upon thy hand, and they shall be for frontlets between thine eyes." — Deuteronomy 6:8 (ASV)

And you shall bind them for a sign upon your hand
As a man ties anything to his hand for a token, that he may remember something he desires; though the Jews understand this literally, of binding a scroll of parchment, with this section and others written in it, upon their left hand, as the Targum of Jonathan here interprets the hand:

and they shall be as frontlets between your eyes ;
and which the same Targum interprets of the Tephilim, or phylacteries, which the Jews wear upon their foreheads, and on their arms, and so Jarchi; of which (See Gill on Matthew 23:5).