Read the dialogue below carefully and answer the question!

Lily : Hi, Ron. How are you doing today?
Ron  : I'm feeling a lot better. Thanks for visiting, Lily. The doctor said I can go home in two or three days!
Lily : That's wonderful! Here, I brought you an apple pie.
Ron  : Wow, that's my favorite!
Lily : Remember to just take it one day at a time and in no time, you will be completely healthy and smiling once again.
Ron  : Thank you so much, Lily!

Lily said the expression "to take it one day at a time". What does the expression mean?