If you have ever played Minecraft multiplayer, you might know that the servers you join are hosted on a computer running a program, called Minecraft Game Server.
While the Minecraft Game server is written by the creators of Minecraft, Mojang, Epic Games doesn't provide an equivalent for Fortnite.
By exploiting the Fortnite internals, though, it's possible to create a game server just like in Minecraft: this is in easy terms what Project Reboot does.
Some Fortnite versions support running this game server in the background without rendering the game("headless"), while others still require the full game to be open.
Just like in Minecraft, you need a game client to play the game and one to host the server.
By default, a game server is automatically started on your PC when you start a Fortnite version from the "Play" section in the launcher.
If you want to play in another way, for example by joining a server hosted by one of your friends instead of running one yourself, you can checkout the "Multiplayer" section in the "Play" tab of the launcher.