The command interpreter that reads what you type and asks the kernel to act on it.
A shell is just another program — not part of the kernel — whose entire job is to read a line of text, figure out what you meant, and turn that into system calls (usually via fork() and exec() to start new processes). It's the layer between you and the kernel.
Linux ships with several shells (bash, zsh, dash, fish, ksh) that share a common core vocabulary but differ in scripting features, interactive conveniences (autocomplete, prompts), and POSIX compliance.
Every shell runs the same basic loop: read a line, parse it into words (respecting quotes, expanding variables and globs), decide if the first word is a builtin (like cd, handled inside the shell itself) or an external command (found via PATH and executed as a new process), run it, wait for it to finish, and print the next prompt.
Shells can run interactively (you type, it responds) or as an interpreter for scripts (a file of commands executed top to bottom). The same shell binary does both — a script is just a shell reading its input from a file instead of your keyboard.
#!/bin/sh but using bash-only syntax like [[ ]] — on distros where /bin/sh is dash, not bash, this fails silently or behaves differently..bashrc by default.