The horizontal line test is a method used to determine if a function is one-to-one, meaning it has a unique output for every input. To perform this test, you draw horizontal lines across the graph of the function. If any horizontal line intersects the graph more than once, the function is not one-to-one. This test is particularly useful in the study of functions in mathematics. A one-to-one function has an inverse that is also a function, making the horizontal line test an important tool for understanding inverse functions and their properties.