When you see your mother (or by extention, another woman) in the afternoon and you want to enquire about how her afternoon is going. It's essentially just a greeting.
I wrote ' how is your afternoon mama' and it said i was wrong! This should be correct
The main dictionary definition of "mchana" is daytime (vs night time), and an evening greeting of "Za mchana?" or "Habari za mchana?" meaning, "How have you been today?" Of course, if it is afternoon time, and someone asks, "Habari za mchana?" I think it would be safe to assume they are meaning something like, "How are you this afternoon?" But I think another English translation could be, "How are you today?", even though it is using 'mchana', not 'leo'.