When grass is cut and dried in the sun it is called hay. Farmers preserve it as food for cattle and horse. In cold countries sunshine is rare. Once there is sunshine, the farmer mows and dries the grass. If he does not do it in time, the grass may be spilt, because there is no knowing when the sun will appear again. It will be loss to him. The proverb thus means that if we get an opportunity of doing a thing, we should readily seize that.
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The future is uncertain. It may be of no use then. We should not therefore miss an opportunity, hoping that we shall get another soon. Idle students lose opportunities. They waste time. When an examination approaches, they find they are not prepared and there is no time. They fail miserably. We should therefore miss no opportunity but fully make use of it as it comes to us.