it means that there's a protocol there, at the heart, for having multiple parties cooperating. it means anyone is free to build their own server or client. it meams we're not all trapped in one walled garden, one silo, it means the freedom to innovate & evolve.
right now there is not a lot of models for how we the peoe can host things online in an interoperable way. we can still use http & the technology that we have (unlike block chain), but we can use well defined vocabulary (https://www.w3.org/TR/activitystreams-vocabulary/#activity-t...) & protocols as a base to communicate & interweave social networking systems.