Halstead Length is a software metric used to measure the complexity of a program. It is calculated based on the number of unique operators and operands in the code. The formula helps quantify how long the code is, which can indicate its maintainability and readability. This metric is part of the Halstead Metrics, developed by Maurice Halstead in the 1970s. By analyzing Halstead Length, developers can gain insights into potential issues in the code, such as difficulty in understanding or maintaining it, ultimately leading to better software quality.