In the fourth aliyah Moshe asks G-d, that if Moshe is finding favor in G-d's eyes at this time, may he request to be shown G-d's glory. G-d agrees, but limits what Moshe sees, to the extent Moshe is able to bear. G-d doesn't show Moshe His Face because no one can survive seeing G-d's Face. G-d says that Moshe will only see His Back. Rashi says the "Back" that Moshe saw was the knot of the tefillin of the head, that G-d wears.
Rabbi Shimshon Raphael Hirsch explains this concept as follows: the "Back" means that we often can not see Divine Providence in something that happens while it is happening, but we can see it later, when looking back at the situation.