Proxies

Minecraft Proxies are servers that will redirect a player to a "backend" server on which they can play normally, acting as a middleman for the client>server connection. They'll stay connected through the proxy the whole time, and that connection can be used to "switch" the player between other servers also connected to the proxy.

When talking about systems, we use a term called "topology", which refers to the structure or shape of the system and it's various elements. IE, the basic topology of a standard server with no proxy would be:

Client -> Server

In the case of a proxy however, the topology changes a bit.

             Client
               V
             Proxy
               V
[Server 1 | Server 2 | Server 3]

With the proxy, the "Client", or player, connects to the proxy which can then connect them to one of the servers it's connected to. As mentioned, this allows the player to switch between each server, but it also grants the proxy some control over each of the servers. That means that plugins running on the proxy can effect all of the servers it's connected to through things like chat, the tab menu, server authorization, ect.

While proxies can be connected to by players and run plugins, they do not have their own playable world and they have their own plugin system. They require a separate server for players to play on, often referred to as a "backend" server, that will only be connected to through the proxy. Their plugins are also different than the plugins you'd run on server software like Spigot/Paper in that they're primarily limited to controlling the player's connection and the information sent over it.

For more information on the types of proxies, their specific limitations, and how to set them up, check out the other pages in this category.