The AGPL requires that you make the program's source code available to anyone who uses it, even if they use it by accessing it remotely rather than by you giving them a copy of the binaries. The SSPL extends that requirement to include the source code of every other program at all involved with the system hosting the software. For example, if you use a closed-source program to back up the data on a server, you may not use that server to host any SSPL software for third parties.