BMP is Microsoft's original bitmap format from the late 1980s. It stores raw pixel data with minimal compression, which makes BMP files enormous (a 1080p screenshot is about 6 MB as BMP, 200 KB as PNG). The format remains common in Windows-internal contexts (clipboard, certain printer drivers, older industrial software) but is rarely chosen as a delivery format today.
How to open
Universal, every OS, browser, and image editor reads BMP. The format is so old it predates compatibility concerns.