Looking at this quote with fresh eyes, I'm now drawn to the word "meaning" and how it relates to action rather than philosophy. Tagore doesn't say that contemplating the future helps us understand life's meaning - he specifically points to the act of planting, of doing something concrete and physical.
This suggests that meaning isn't found primarily in thought or meditation, but in purposeful action. It's not enough to theoretically appreciate future generations - we must actually do something that benefits them. The physical act of digging soil, placing a seed, and tending to a young tree becomes a form of embodied wisdom.
I'm also struck by how this contradicts many common notions of life's meaning that focus on personal achievement, happiness, or self-actualization. Instead, it suggests that meaning emerges when we transcend our individual existence and connect ourselves to the greater flow of time and generations. The tree planter has begun to understand something profound precisely because they've moved beyond self-interest.