A decentralised network means that no single computer system acts as the point of contact for the network services to function across the network. The network isn't a single web page that you connect to or a single database or designed to run off a single server; it is all the computers and websites combined together, all enabling the network to run in the background and each maintaining their own information and providing their own set of services. Here the shared network protocol provides the common context that they can organise around, and the shared network services provide common means of communication within that context.
One of the main aims of the Spring network design is for it to continue to run and sustain itself just by people being connected to it -- anyone running the software will have the ability to maintain their region of the network and if necessary delegate others nodes to do the same, all performed automatically in the background as part of the network protocols. A self sustaining network means that if our servers go dark, even permanently, we don't pull the network down around us -- it repairs itself.