I personally had a lot of bad grades and W's. Most of the schools I was accepted to had some secondary asking me to explain. Every one of those schools asked about it in the interview and mentioned the secondary responses.
From what I've gathered on the interview trail, writing those responses in a clear-cut manner and owning up to mistakes AND demonstrating growth is what needs to happen. Even though my circumstances could have easily excused poor performance, I made sure to detail what I did to overcome those circumstances and strengthen my fortitude. Then demonstrated how those lessons helped me do better in my SMP.