A very tangible reason that phrase resonates with me is that one of my favorite things to do is to capture sunlight streaming through the trees when I take photos. There is something magical, even divine about it to me.
I also like the phrase because when light streams through a canopy of trees there is often just a hint of something more; something much larger than me. Mary Oliver expresses it so perfectly as a "hint of gladness."
I have discovered that it takes a bit of awareness to feel and experience a "hint of gladness" and I believe this type of noticing enriches, and can even transform our lives. Often, these hints are more than noticing sunlight streaming through the trees; they awaken us to morsels of spiritual wisdom.