The last Posuk of this week's Parsha describes Tefillin as a sign to be worn on one's hand, and as a "frontlet" to be worn between the eyes, because Hashem brought us out of Mitzrayim with a strong hand (שמות י"ג;ט"ז). The Gemara in Menachos (דף ל"ז.) explains that the Tefillin Shel Yad must be covered, since the Torah in an earlier Posuk (שם פסוק ט) says "והיה לך לאות על ידך," "it shall be for you as a sign on your arm". This is understood to mean that the Tefillin Shel Yad must be a sign for you, the one wearing it, and not a sign for others. In his final comment on our Parsha, however, Rashi writes regarding the Tefillin Shel Rosh that anyone who sees them bound between the eyes will recall the miracle (of Yetzias Mitzrayim) and will speak of it. Apparently, the Tefillin Shel Rosh indeed should be seen by other people.
We must therefore understand why this differentiation is necessary. Why must the Tefillin Shel Yad be a sign solely for the individual wearing them, whereas the Tefillin Shel Rosh are a sign for all who see them? It seems that both of these aspects of Tefillin, the hidden sign and the revealed sign, are necessary and, in fact, complement each other. Rav Moshe Feinstein explains that in order to inspire others to remember the miracle of Yetzias Mitzrayim, the person himself must remember this incident and realize its everlasting significance. One cannot preach to others what one doesn't believe in or practice himself and hope to be successful. The Gemara in Sanhedrin (דף י"ח.) thus says that one must reprove himself before he reproves others. This idea, Rav Moshe Feinstein suggests, is hinted at by the two different lessons of the Tefillin. Once a person will himself see the private sign of the Tefillin Shel Yad and be inspired thereby to act properly, he can then, and only then, influence other people to follow his lead by letting them see the revealed sign of his Tefillin Shel Rosh.
We must all understand this important message of the Tefillin and apply it in our daily lives, in order to improve our own relationship with Hashem and, hopefully, be able to positively influence others. The best way to teach anyone anything is through example. If a person lives an immoral, decadent life, all his fiery and powerful speeches about morality and ethics will have little impact. If, however, one is himself a model of kindness and ethical behavior, he will then be able to influence many others to follow in his footsteps.