Moses continues reminding the people of Israel what the previous generation of Israelites did. Yahweh is faithful to the covenant he made with Abraham and is also faithful to the promises he made with Esau and Lot. Because of this, Israel was given specific land it was able to conquer in Canaan.
Possible meanings are 1) the Israelites traveled around the mountain called Seir for a very long time or 2) the Israelites wandered in a region called Mount Seir for a very long time.