The quote suggests that a gentle rainfall can have a significant impact on the appearance of the environment, specifically on the grass. When it rains, the water provides the necessary moisture for the grass to grow and thrive, making it appear greener than before. The gentle nature of the rain is important because it allows the water to be absorbed into the soil gradually, which is better for the grass and other plants than a heavy downpour that could potentially damage them.
Metaphorically, the quote could also mean that small positive changes or actions can have a significant impact on a larger scale. Just as a single gentle rain can make the grass greener, small acts of kindness or positive changes in behavior can lead to larger positive outcomes in our lives and in the world around us.