BH i very often have the merit of putting tefillin on jews that have never put it on. how do i describe to them what it does spiritually?
For the uninitiated I would start with Rav Yosef Karo's explanation for the placement of t'filin: One goes next to the brain to represent one's intellectual recognition and obedience of God and one goes next to the heart to represent one's emotional recognition and obedience of God. (Source - Orach Chayim 25:5)
You should also tell them what it does for you, personally, when you put them on.
Chances are, you don't know what that is. After you pay enough attention to be able to explain to someone else what it does for you to put them on, you will probably find you are much more successful at getting others to do so.