What is a "Serendipity Drop"?

Serendipity / Mystery Drops are one of the cooler things you can do with the swaps. If you have
spare seeds and feel like adding a little more serendipity into your corner of the world, why not
leave a packet somewhere random – and see who picks it up? You could leave your seeds in a park, at
your local allotment, a gardening show, local garden centre….anywhere we’re there might be a potential gardener walking by!