Basically its a daughter card for the RPi zero that can convert it so it has the full footprint of a RPi. Board runs around $15, so 15 + 10 for the zero (if you can find one) is a bit cheaper than buying a RPi3 these days.
https://www.hackster.io/news/spotpear-s-adapter-boards-bulk-out-the-raspberry-pi-zero-family-to-match-the-raspberry-pi-3-4-f3cf4d96008c#amp_tf=From%20%251%24s&aoh=16668731407917&csi=0&referrer=https%3A%2F%2Fwww.google.com&share=https%3A%2F%2Fwww.hackster.io%2Fnews%2Fspotpear-s-adapter-boards-bulk-out-the-raspberry-pi-zero-family-to-match-the-raspberry-pi-3-4-f3cf4d96008c