SSPL explicitly states that anybody who wants to offer MongoDB as a service — or really any other software that uses this license — needs to either get a commercial license or open source the service to give back the community.
Looks like a pretty straightforward extension of GPL principles, by replacing the linking of licensed code to remotely calling the licensed code.
It can have a lot of implications for service providers and commercial developers alike, depending on the way commercial licenses will work. (Some DB licenses have pretty stifling clauses, like Oracle's or MS SQL's.)