The Mezuzah is affixed to all the doorposts of a Jewish home (except for the bathroom) as a reminder of the covenant between G-d and the Jewish People. Hanging a mezuzah on one’s door is a Biblical commandment and a Jewish tradition to this day.
The car mezuzah is designed like a mezuzah that would go on the doors of your home, but rather than containing the text of the Shema, from Deuteronomy, it has the traveler’s prayer. These car mezuzahs come in a variety of styles set to look like door mezuzahs, with the traditional letter shin, different messages, or other Jewish imagery.