Rust is an expression-oriented language, which means that most pieces of code are in fact expressions, that is, they compute a value and return that value (in that sense, values are also expressions). However, expressions by themselves do not form meaningful code; they must be used in statements.

The let bindings like the following are declaration statements; they are not expressions:

// see Chapter 2/code/expressions.rs
let a = 2;    // a binds to 2
let b = 5;    // b binds to 5
let n = a + b;   // n binds to 7

However, a + b is an expression, and if we omit the semicolon at the end, the resulting value (here 7) is returned. This is often used when a function needs to return its value (see examples in the next chapter). Ending an expression with a semicolon like a + b; suppresses the value of an expression, thereby throwing away the return value and making it an expression statement that returns the unit value (). A code is usually a sequence of statements, one on each code line...