A mathematical expression is a combination of numbers, variables, and operators that represents a value. It can include elements such as addition, subtraction, multiplication, and division. For example, the expression 3x + 5 includes the variable x and shows how to calculate a value based on different inputs for x. Mathematical expressions can be simple, like 2 + 3, or more complex, involving multiple operations and variables. They are used in various fields, including science, engineering, and finance, to model relationships and solve problems. Understanding these expressions is fundamental to learning algebra and other areas of mathematics.