What does Jeremiah 3:6 mean?

During King Josiah’s reign, the Lord said to me, 'Have you seen what Israel, that unfaithful nation, has done? She’s gone up on every high hill and under every green tree, acting like a prostitute there.' Jeremiah 3:6 - Modern Text Bible

(God points out Israel’s widespread idolatry during the time of Josiah, using the metaphor of prostitution.)