Don’t feel bad you don’t remember, I think fandom collectively forgets it or doesn’t like to remember. But here’s the relevant panels— this conversation happens after Misa is approached by Rem again during the Yotsuba arc and has to remember she and Light are Kiras. Rem says she can no longer kill L because doing so may lengthen Light’s life, and since she has feelings for Light now, that would kill her.

Rem’s opinion on Light has changed since they first met, and as she highly values sacrifice, she sees him as ‘pure’ as Misa now. (Poor misguided shinigami on both accounts)

This raises all sorts of interesting issues like who exactly got Rem’s lifespan when she did finally kill L. I’m in the minority believing it was split between Light and Misa but that’s the most logical conclusion.