The term "expression" means a computer program statement that evaluates to some value. A Boolean expression would evaluate to either true or false, such as ( money > 100 ). An assignment statment will have a variable, followed by an equals sign, followed by an "expressions" that computes a value to store in the variable.
Most computer programs follow a sequence of steps, choosing what to do next based on "boolean expressions" and the current "state" of the program.