Jackson is not actually in any legal trouble, because he didn't actually do anything, but according to taped conversations, corrupt former Illinois governor Blago seemed to think he might end up with a pretty decent payday if he appointed Jackson to the Senate to replace Barack Obama.
Blago told his brother to get in touch with Raghuveer Nayak, a friend of and fundraiser for Jackson, and try to sell Nayak on giving Blago "tangible political support" in exchange for the Jackson appointment. Now Blago didn't actually seem to want to appoint Jackson, 'cause Jackson had not really been his best political ally, but it still seemed worth a shot to approach Jackson's rich friends. And then there was this fundraiser Jackson and his brother and Nayak went to, a couple weeks before Blago's arrest, but honestly who knows what was actually going on out there, in Chicago, with the favor-trading.
Now the "Office of Congressional Ethics" is a toothless citizens' board sort of thing that was just formed last year, and all they can really do is ask politely for interviews and documents and then recommend that the for-real Ethics Committee look into something, but still: you don't really want to be the only member of the Illinois congressional delegation not named "Roland Burris" to be under investigation by any group with "ethics" in the name.