A rational expression is a fraction where both the numerator and the denominator are polynomials. These expressions can be simplified, added, subtracted, multiplied, or divided, similar to regular fractions. However, it is important to note that the denominator cannot be zero, as this would make the expression undefined. Rational expressions are often used in algebra to solve equations and model real-world situations. They can represent relationships between quantities, such as speed or density. Understanding how to manipulate these expressions is essential for success in higher-level mathematics, including topics like algebra, calculus, and pre-calculus.