A login system is the gate a server uses to confirm your identity before you can actually play. Instead of trusting only your Minecraft session, you create an in-game password and authenticate each time you connect. Most servers hold you in a small lobby, a spawn cage, or a frozen state until you log in, then you drop into survival, hub, or whatever mode the server runs.
You see this most on offline-mode servers and networks that run their own account layer. The point is simple: stop someone from joining with your username and walking off with your inventory, claims, or ranks. When it is set up well, you barely notice it after day one. When it is not, it is constant friction: blocked commands, chat spam, and getting timed out while you are still typing.
The flow is usually /register on first join, then /login after that. Some servers add recovery options or extra checks when your IP changes. On a proxy network, the best experience is logging in once and then moving between subservers without re-authing every time.
The feel is more old-school community server than modern account-trust. You are responsible for your account security, and staff often cannot recover your password unless you set up a reset method. It can also affect cosmetics: depending on how the server handles profiles and forwarding, skins may not behave like they do on standard online-mode servers.
