The instructions that tell the computer to do something in a Rust program are almost all expressions. An expression tells the computer how to compute a particular value, and produces that value as its result. In math, 2 + 2 is an expression with a resulting value of 4. Similarly, (2 + 2) - 1 is an expression with a resulting value of 3, which is itself made up of an addition expression and a subtraction expression. In Rust, the same basic idea applies: expressions tell the computer how to find a value, and they can be combined together, because using an expression that produces a value has the same result as using that value directly, just as writing (2 + 2) - 1 has the same result as writing 4 - 1.

Not all expressions in Rust look like math, though. Rust is a programming language, not just a calculator. It's the idea of expressions, which combine values to produce new values, that matters.