HLSL, or High-Level Shading Language, is a programming language developed by Microsoft for writing shaders used in 3D graphics applications. It allows developers to create visual effects and manipulate how graphics are rendered on the screen, making it essential for game development and other visual media. HLSL is primarily used with the DirectX graphics API, enabling programmers to write code that runs on the GPU (Graphics Processing Unit). This helps achieve high-performance rendering and complex visual effects, enhancing the overall quality of graphics in video games and simulations.