What does Genesis 49:17 mean?
Each of the tribes of Israel is descended from one of Jacob's twelve sons (Genesis 35:10–11, 23–26). His deathbed oracle about the future descendants of these sons (Genesis 49:1–2) has brought him to Dan (Genesis 49:16). Here, the small tribe of Dan is described as a snake that ambushes a horse, knocking its rider off when the horse reacts.This statement is unclear, and heavily debated. Two interpretations are commonly put forward. One is more positive, suggesting Dan, though small, will still do its share of damage to Israel's enemies. In fact, Samson was from Dan and served as one of Israel's judges. He repeatedly defeated their enemies, the Philistines, in various battles (Judges 13—16). Also, the people of Dan attacked and defeated the town of Laish (Judges 17—18). Comparison to a waiting serpent, in that case, is a compliment about a relatively small being that is capable of great effect, thanks to skill and strategy.
It's also possible to read Jacob's description of Dan's descendants in a negative light. Snakes are often associated with sneakiness and falseness. Dan's descendants will be some of the first to delve into worshiping idols, which brought on the people the wrath of God (Judges 18:27–31).
A more obscure interpretation considers the association of Satan with the serpent in Eden (Genesis 3:1) and this passage's end-times implications (Revelation 5:5; 20:4–6). Likewise, Dan, as a tribe, is not mentioned as part of end-times Israel (Revelation 7:4–8). This leads some to suggest the "beast" of Revelation (Revelation 13:1), the Antichrist, could be from the tribe of Dan.