Care : se soucier, s'intéresser. "she cares about the poor: elle se soucie des pauvres - I don't care: ça m'est égal - I didn't know you cared : I didn't know you were interested or it was important to you.
You better say : What does "I didn't know you cared" mean?
Thank you Jean-Pierre. Obviously 'what means' is a very bad grammatical mistake. Honey Liz