A Shell builtin is a command or a function, called from a shell, that is executed directly in the shell itself, instead of an external executable program which the shell would load and execute.
Here are some builtins provided by Bash, the default shell in Debian. See the bash manual page:
cd - change the shell working directory.
alias - create a command alias
echo - write text/arguments to the standard output
First released in 1989, it has been used widely as the default login shell for most Linux distributions. The Bourne Shell is an early and important Unix shell written by Stephen Bourne and distributed with Version 7 Unix circa 1978. Bash was created in 1987 by Brian Fox. In 1990 Chet Ramey became the primary maintainer.
Bash is the default shell on Debian and most Linux systems.
bash package in Debian
bash manual page in Debian
Upstream specific information
https://wiki.archlinux.org/index.php/Bash - Bash on Arch Linux Wiki
http://www.gnu.org/software/bash/manual/bashref.html - Bash Reference Manual
https://www.tldp.org/LDP/abs/html/abs-guide.html - Advanced Bash-Scripting Guide
http://linux.org.mt/article/terminal - An introduction to Bash
http://www.tldp.org/LDP/Bash-Beginners-Guide/html/index.html - Bash Guide for Beginners
https://gna.org/projects/mbfl - Marco's Bash Functions Library