What does "lean" mean here?
In this context, the adjective "lean" is used to mean an activity or a period of time involving little nourishment or substance. For wild animals, there is a substantially lower amount of food they can consume, so "lean" is often used to describe the winter months. Ex: The squirrel is saving food for the upcoming lean winter months. Ex: The winter months to come will be lean, so all the creatures in the forest are storing up food.