RE: [math-fun] Algebra I (RiP)

Bill Gosper writes

Seriously, to sum up, at the end of a course taught from this book, I doubt anyone could answer the question "What is algebra?".

Well, I haven't taken the course but even so I don't know how to answer this question. I'd be most interested in anyone else's answer. We're certainly not talking about "modern algebra" a la Birkhoff-MacLane, so what is "high school algebra" about? I think of it as where you learn to say "let x be ..... " Any other ideas?