“bind them for a sign on your hand.” This verse is the basis for the wearing of phylacteries, little boxes that contain scripture that the ultra-orthodox Jews tie on the back of their hands and also tie on their foreheads. However, God did not mean for Deuteronomy 6:8 to be taken literally that way, which we can tell from the fact that this rule is for all Israel, and the nature of daily life in ancient Israel would not accommodate it. By saying to bind them to your head and hand He was emphasizing that the Word of God should be near their thoughts (head) and in what they do (hands).
The pure nature of this command of God was occasionally perverted by religious superstition, as we can see by the very word “phylactery,” which comes from the Greek word phulassō (#5442 φυλάσσω) which means to guard, to keep watch, to protect you from a person or thing, to keep safe. Thus the very thing that God said to assure that people would keep His Word and keep people from wandering from it occasionally became an object of superstition, complete with all the rules and regulations about exactly how to tie it on, when and where to wear it, etc.
[For more information, see commentary on Matthew 23:5.]