Basically, it is just either planting a tree for someone, or helping out in another persons garden. For example, pulling out weeds, or clipping flowers to give to them. It could be for an elderly person or a best friend.
Get some potting mix from your local hardware store, then dig a hole in the ground, and sit a plant or seedling (really cheap at hardware stores) in it. Fill the rest of the hole, and water thoroughly. Or you can get some secateurs, clip some flowers and give them to someone special. It's doing a favour to the plant, as more flowers will come next time, and it is an easy way to make someones day.