The EDID is a small binary blob your display sends to your system that describes the capabilities of the display, for example, what physical dimensions it is, or what resolution is supports at what clockrates.
I recently had to modify my EDID to fix a DTD (Detailed Timing Descriptor), which caused the display to report a 59.93 Hz refresh rate mode that actually could only do about 48 Hz. Nudging around the pixel clock and V-Blank "fixed"