I know you're itching to dive right in to Common Lisp, so let's get started!
↑, ↓ to navigate evaluation history.
⇧ Shift + ↲ Return to continue your expression on the next line.
↲ Return/Enter to evaluate what you've entered.
The REPL runs in your browser, never communicating with the server. The history is stored on your computer.
Every piece of code you write in Lisp is an Expression. An expression can either be an atom or a list, and always returns a value when it is evaluated.
At the prompt, type: (+ 1 1) — that expression is a list. Each element of the list is also an expression. It evaluates to, or "returns", 2, the result of passing the arguments 1 and 1 to the addition function, +.
Now type: 2 — this expression is an atom. It self-evaluates: i.e., it returns itself.