A shell is a program that takes user input from a keyboard and passes it as commands to the operating system.
Most Linux systems use bash, which is an acronym for Bourne-Again Shell. It’s an enhanced version of sh, which was Unix system’s original shell program. There are other shell programs like zsh.
Within the context of cybersecurity, the term getting a shell on a system means that the host been compromised and the attacker has obtained shell-level access on the system.