A Roblox private server is a separate game server controlled by the person who created or pays for it. It is sometimes called a VIP server.
Private does not always mean one person only. The owner can usually invite connections or allow selected users, depending on the game and account settings.
How private servers work
If a game supports private servers, its page includes a Servers section where you can create or join one. Some are free, while others renew for a Robux price chosen by the game creator.
The server owner can change its name, decide who may join and generate a new link. Exact controls depend on Roblox age settings and the game itself.
What a private server changes
You avoid being placed with random public players, which is useful for playing with friends, recording a video or practising. The game's normal rules still apply.
A private server does not give the owner permission to exploit, harass people or ignore Roblox Community Standards. Game moderators may still be able to review reports and server data.
Be careful with invite links
Only open server links from somebody you trust, and check that the address really belongs to roblox.com. A fake private-server link is a common way to steal login details.
You should not need to install an extension, paste code into your browser or send a security code to join. Close the page if it asks for any of those things.