What Is the Horizontal Line Test?

The horizontal line crosses the graph depending on the number of outputs that exist per input.

The horizontal line represents a single $x$-value, and if it crosses the graph at multiple $y$-values, the function has too many outputs per input.

The horizontal line test can determine how many values in the domain exist per one specific value in the range, which tells if there is a one-to-one ratio between outputs and inputs.