Here's a breakdown:

* Hay: In the past, hay was essential for feeding livestock during the winter.

* Sun: Sunlight is crucial for drying hay.

* Shine: This refers to a good weather window, when the sun is out and the weather is favorable for haymaking.

The proverb's meaning: If farmers waited for the perfect moment to make hay, they might miss the chance entirely. The weather can change quickly, and if it rains, the hay will become unusable. So, farmers need to seize the opportunity when the sun is shining and make hay while they can.

In a broader sense: The proverb encourages us to be proactive and make the most of favorable situations. Don't wait for the perfect time, because it might never come. Take action when you have the chance, because opportunities are often fleeting.