Linux built-ins are commands that are built into the shell, much like shelves that are built into a wall. You won’t find them as stand-alone files the way standard Linux commands are stored in /usr/bin and you probably use quite a few of them without ever questioning how they’re different from commands such as ls and pwd.
Built-ins are used just like other Linux commands. They are likely to run a bit faster than similar commands that are not part of your shell. Bash built-ins include commands such as alias, export and bg.
As you might suspect, because built-ins are shell-specific, they won’t be supplied with man pages. Ask man to help with bg and you’ll see something like this: