This verse sets the scene for a pivotal moment in the story of Moses and the history of Israel. Let's break it down:

"Now Moses was keeping the flock of Jethro, his father-in-law, the priest of Midian..." This establishes Moses' current life. He's living in exile in Midian, working as a shepherd for his father-in-law, Jethro (also known as Reuel). Jethro is identified as a priest, which may suggest a connection to a different religious tradition than the one Moses will eventually lead the Israelites to.

"...and he led the flock to the back of the wilderness..." This indicates that Moses was going to a remote and less-traveled area to graze the sheep. It suggests he was venturing away from settled or cultivated lands. The phrase "back of the wilderness" may also have a metaphorical implication, suggesting Moses was also at a point in his life where he felt far removed from his previous identity as an Egyptian prince and perhaps felt forgotten or overlooked.

"...and came to God's mountain, to Horeb." This is the crucial part. Horeb is another name for Mount Sinai. This is where God will reveal himself to Moses and commission him to lead the Israelites out of Egypt. The significance is that a seemingly ordinary task (shepherding) has led Moses to a holy place where a life-changing encounter will occur. The verse hints that even in the mundane aspects of life, one can stumble upon divine encounters. It suggests that God's presence can be found even in unexpected places, like a remote mountain in the wilderness.

In summary, this verse serves as an introduction to the burning bush narrative. It highlights:

Moses' humble circumstances: He is a shepherd in exile.
The setting: A remote wilderness area near Mount Horeb.
The stage for divine intervention: This verse sets the scene for God's revelation to Moses, which will alter the course of history.