This is an old word for “mile” or “mile post.”
You’ll recognize tsin from March, meaning wood or post in this case.
Sitą́ is one of the position words, which in the Navajo language are specific to the object’s characteristics. Since the post is a slender and stiff object, sitą́ is appropriate. It is a declaration that the object is “in place” or “it is set there.”
So together, the phrase literally means wood (post) that is set. Some parts of the Navajo Reservation actually have kilometer posts now, so this may be confusing in some parts of the rez.