I found a blog post (https://safing.io/blog/2022/09/06/spn-vs-vpns/), but you have to go fairly far down the page (to the header "Cryptographic Identity Protection") to begin to get the gist of what it is.

"This was originally invented for Tor and is called Onion Routing. This way, every server in the chain only knows the previous and the next hop. No server ever knows who you are AND where you are going to."

"As VPNs are centralized, all their servers are operated by only one entity - the VPN provider itself. They can, therefore, monitor all you traffic and see what you are up to. This is why they tout their “No Logging” policies so loudly, because they know they can see everything."

"SPN on the other hand invites the community to join the network and strengthen it by adding diversity to the operators of the network. This way - in addition to the cryptographic protections - it is made almost impossible that anyone will ever be able to track you through the SPN. Also, because of the cryptographic identity protection mentioned before, there are not even any logs with identifiable data to begin with!"

It sounds like it is a next-gen VPN service which addresses the shortcomings of the current VPN services by splitting the service into relays and exits.