What does Judges 3:6 mean?

This verse completes the reintroduction begun in Judges 2:6. The writer is summing up how Israel failed to obey the Lord once the people settled into the land of Canaan. They left many areas unconquered, and the existing kingdoms there would become a source of raids and oppression in the years to come (Judges 3:3). In the previous verse (Judges 3:5), Scripture emphasized that the Israelites settled among the peoples of the land instead of driving them out, as God commanded (Deuteronomy 20:16–18).

Here, it's pointed out that the Israelites intermarried with the people of the land. They gave their Israelite daughters to be married to Canaanite sons and took Canaanite daughters for their own sons. This was another direct violation of God's commands to Israel:

"You shall not intermarry with them, giving your daughters to their sons or taking their daughters for your sons, for they would turn away your sons from following me, to serve other gods. Then the anger of the Lord would be kindled against you, and he would destroy you quickly" (Deuteronomy 7:3–4).

This intermarriage was not a concern about ethnicity, or race. Rather, as explained here, widespread intermarriage was a sign of the two cultures blending. Despite God's warnings, Israelites served the gods of the Canaanites and adopted their immoral and depraved worship practices. These practices included having sex with temple prostitutes and even sacrificing children on altars. What follows in the book of Judges are the consequences of Israel's association with deep, moral evil.