What is DOS?
DOS (short for Disk Operating System) is a family of operating systems for IBM-compatible computers, very popular in the eighties and most of the nineties. One of the most used DOS systems was MS-DOS (MicroSoft DOS) obviously developed by Microsoft.
Starting with Windows XP, MicroSoft pure DOS software became incompatible with Windows operating systems. Old DOS games were difficult to setup to run properly on modern computers. That's why someone decided to create DOSbox.