This wasn’t a meeting about “anything” — it was a meeting to obtain information on Hillary Clinton to smear her. And Junior had the knowledge that Russia was specifically interested in supporting Trump and influencing the election on his behalf, as evidenced by the emails HE shared with the world.
So when Junior became aware that A) Russia wanted his dad to win the election (and this desire came from the “highest levels” of the Russian government, and B) actively planned to influence the election by using him as a tool to do it, did Junior go to the FBI or any other US agency and say, “I have information that the Russians are planning to meddle in our election”?
No, he didn’t. Instead he responded, “love it” to the Russians and eagerly went to participate in their scheme. Making him an accessory to their work, even if he didn’t get the information he was promised.
Do you really need to have the meaning of “collusion” explained to you?