There's a bit of debate, given the specifics. Going by the short version, campaign law forbids accepting or soliciting foreign money or other notable aid from foreign sources. Trump Jr. was fully intending to break that, given the e-mails, and Manafort and Kushner were apparently in on it directly. Fairly certainly Trump, himself, too, given his statements right after the e-mails. Supposedly, nothing of value was actually presented at the meeting itself, though, which gives a tiny bit of a defense regarding whether campaign law was actually directly broken or not.