OpenGL (Open Graphics Library) is a multi-platform graphics API - a set of specifications that form a common "language" between applications (games, 3D programs) and the graphics card (GPU). Its main advantage is multi-platform: the same code can run on Windows, macOS and Linux. Although it is increasingly being replaced in modern games by newer APIs such as Vulkan - a low-level, cross-platform interface developed by the Khronos Group consortium that gives developers much more direct and efficient access to hardware - or the proprietary Direct3D 12 (Windows) or Metal (macOS/iOS), OpenGL remains crucial in many professional applications (CAD, DCC, scientific visualization).
Unlike transmission interfaces (HDMI, DisplayPort) that just send the finished image, OpenGL describes how the GPU creates the image - from geometry to shading to final rasterization - i.e. the complete graphics pipeline, the sequence of steps through which the data passes: First vertices are prepared (vertex processing), then visible polygons are determined (clipping), then they are turned into pixels during rasterization, and finally fragment shaders and post-processing are applied before the final signal is ever sent down the cable to the monitor.
Since version 4.4 (2013), OpenGL has focused on big data matching - introducing sparse textures that can stream giant details without taking up all the memory, and "immutable" buffers that reduce allocation overhead. Version 4.5 (2014) adds Direct State Access so you can set objects directly without constantly switching context, making multi-threaded rendering much easier. The latest 4.6 (2017) converges with Vulkan: it supports the universal SPIR-V binary shader format, allows you to optionally disable error checks for even lower latency, and offers more detailed pipeline traversal statistics.
