The project idea aims at developing a plugin (as GNU Social is true to the Unix-philosophy of small programs to do a small job) that will implement the ActivityPub Protocol in GNU Social.
GNU Social is a social communication software used in federated social networks. In a federated social network, user data stays in the user's server instead of a centralized one.
In order to achieve said decentralization, a variety of standards on how communication between different pieces of software in a federated context should be done were created, such as OStatus. ActivityPub, however, is the newest and covers parts left out of OStatus's specification, namely the app/client development.
Because of this and given that newer software will focus on the implementation of the ActivityPub Protocol, GNU Social is looking forward to supporting it in order to stay updated and relevant in an even larger fediverse.