SMACSS, or Scalable and Modular Architecture for CSS, is a set of guidelines developed by Jonathan Snook that aims to make CSS codebases more scalable and modular. It addresses the complexity and tangling of CSS as web applications grow by organizing CSS rules into five categories: base, layout, module, state, and theme. This structured approach enhances readability and maintainability, making it easier for developers to manage and debug CSS, especially in large-scale applications. SMACSS emphasizes avoiding the use of "!important" and CSS resets to prevent specificity issues, and it promotes separating CSS into distinct files for clarity. While SMACSS doesn't decrease the running time of CSS, it significantly reduces the time spent maintaining and debugging code, which is valuable in the IT industry. Its principles are considered essential for web developers, helping them to efficiently manage the evolving needs of web applications while ensuring compatibility across different browsers and devices.