What a licence is
A licence is a purchased copy of Minecraft: Java Edition tied to a Microsoft account. It gives you a unique protected name, your own skin, updates and access to servers that verify accounts. Buy only from the official site or official resellers.
online-mode
Servers with online-mode=true accept only licensed accounts and log you in automatically. Servers with online-mode=false accept any launcher and name; a login plugin protects names via /register and /login. "Failed to verify username" means the server requires a licence.
Staying safe on offline-mode servers
Use a unique password per server, register immediately, never type passwords on "free donation" sites, and link email or Telegram for recovery if offered.
Third-party launchers
Many modded servers ship their own launchers. Download them only from the official server website or the link on its Minecraft-Servera page, and avoid "cheat" clients — they are the most common malware source.
Choosing
Licensed players can join any Java server; others should pick servers without the "Licence required" warning or with their own launcher; phone players need Bedrock-compatible servers.