It holds the database name, user and password, the authentication keys and salts, the table prefix and constants that change behavior, including those that control debugging and automatic updates.
Because it contains secrets, exposure of this file is serious. Backup copies with extensions such as .bak or .old in the web root, directory listings, or a file read flaw can reveal it. WordPress also allows it to be located one level above the web root.
Restrict its permissions, never leave copies on the server, and keep it out of repositories. If the file may have been exposed, rotate the database password and security salts, and review the database for unexpected users or content.