In Exodus 6:6-8 we see God's gospel. He will rescue his people out of slavery, with an outstretched arm and through judgement, so that they are redeemed, will know that he is the LORD, will be his people, and will be given an inheritance in the land. This is Christ's rescue of his people from sin's slavery, through the cross where his arms are outstretched and he bears the judgement that is ours, into the family of his people, the sure hope of the new creation in which we 'know the LORD'!
Moses preaches this to his people but, v9, because of their broken-spirit's they do not believe. Will the LORD not then rescue them? No, he will do it! What's next? Moses must go to Pharoah. He protests once more because he says he has 'uncircumcised lips' (unbelieving lips?), nonetheless God give them words. Then comes v14-29 and then Moses protest is repeated in v30 - the narrative hasn't moved on. What's in the middle? A genealogy of Moses and Aaron. Why?
Afterwards in chapter 7 the LORD says to Moses that he will be as God to Pharaoh, and Aaron will be his (Moses) prophet to say what God commands. Pharaoh will be commanded to release God's people, the Egyptians will know that he is the LORD (this is all for God's global fame!) but Pharaoh's heart will be hard and though the LORD is clearly victorious (Aaron's staff swallows up the imitations of the Egyptians, swallowing up death in victory...) the Tyrant will not bow to the LORD.
Why the genealogy? Who's there? We given the 'heads' of the houses, the sons of Reuben, Simeon and Levi. From Levi we find Gershon, Kohath and Marari... then their sons.. including from Kohath, Amram... and Amram's sons by Jochebed are Aaron and his younger brother Moses (v20). And, v25, "these are the Aaron and Moses to whom the LORD said: bring out the people... it was they who spoke to Pharaoh.. this Moses and this Aaron."
This genealogy serves to tell us exactly who these upstarts are who confront the mighty Pharaoh. They are not just any people. It establishes them firstly as part of Israel (though Moses is a long-term exile with a foreign [believing] wife) by listing from Reuben, Simeon and Levi. Secondly, it shows them specifically to be the sons of Amram, son of Kohath, son of Levi (who was, 6v8, son of Jacob son of Isaac, son of Abraham to whom the land of Canaan, three generations to which the LORD had promised the land). This was no random rebellion, they are locked into God's people and God's promise. It is this Moses who doubted Pharaoh would listen, and the LORD told him as much (v4) nonetheless the sentence of death had to be proclaimed against the Tyrant as part of God's salvation through judgement of his people.
These events happened, through This Moses and This Aaron, the story of God locked into space and time as is his ultimate salvation from slavery to sin, which Exodus both points to and proclaims. This Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men...