"just picking on the kitchen" makes no sense. Maybe what you heard was "just picking UP the kitchen". To "pick up (after) something/someone" means "to clean up, put away, organize". I picked up after my dog. I picked up the house after the party. (put glasses and plates away, threw away trash, etc.) I picked up (in) the kitchen. (cleaned up the dishes, put away food, etc.) "picking up the kitchen" is a chore that everyone has to do every day. So if someone asks "what are you doing?" and you say "just picking up the kitchen", it means you are a normal, basic chore. So you can think of it as nothing too important. You could respond with any other everyday task you do: "just cleaning the house", "just watching some tv", "just washing the windows". It means you are not too busy.