**Spoilers**
She was doing some mass murderings for him while he was leading the investigation alongside L. L had handcuffed the two of them together (weird, I know), so Light couldn't do anything he wanted to, so he had Misa do the killings for him so that they would still go on even while Light was locked up and unable to access information about criminals.
So she was like, "I was useful to you, right?"
And he's like, "Yeah yeah, I love you too."
