Halstead Complexity Measures are a set of software metrics that quantify the complexity of a program based on its operators and operands. Developed by Maurice Halstead in the 1970s, these measures help assess the maintainability and understandability of code by analyzing the number of unique and total operators and operands used in a program. The key metrics include program length, calculated program length, vocabulary, and volume. By evaluating these factors, developers can gain insights into the potential difficulty of understanding and maintaining the code, ultimately aiding in better software design and quality assurance.