A fantastic, calming and uplifting way of reaffirming your friendship with others.
While the activity itself is self explanatory, perhaps the larger picture is not: Often the plants that you swap will have been grown from seed or cuttings. The plants may be from your favourite plants or you may have put a vast amount of effort into getting the cuttings to grow.
When swapping plants takes place, people will also swap advice on how to best grow the plants: where to place them, care instructions and anecdotes. Most gardeners I know will swap plants as much for the human interaction as for the plants themselves.
You are not just swapping plants, you are exchanging parts of yourself with others