What is ReplicationGraph plugin?

I got this error while compiling a project wich I downloaded from GitHub: “1>UnrealBuildTool: ERROR: Unable to find plugin ‘ReplicationGraph’ (referenced via AGame.uproject). Install it and try again, or remove it from the required plugin list.”
I loked on internet I found that this plugin is loaded with Fortnite, but I can not found it in UnrealEngine and how to activate it.
Help please, where should I look?

You should use the release branch, not master.