MS-DOS was the most commonly used operating system for any and all computers in operation during the 1980s. Created by Microsoft, it was an operating system that did not feature the graphical interface that has since become a staple of Windows operating system releases from Windows 3.1 onward. DOS featured a command prompt format, which required users to execute a series of commands in the prompt in order to accomplish any necessary tasks. Essentially, a person had to type out exactly what it wanted their computer to do (in the format that DOS understood) in order to get anything done. Microsoft abandoned work on MS-DOS in 2000.
MS-DOS would examine files and programs on a computer in three separate ways in order to complete any command a person needed it to do. One way was that DOS would look at the file called "command.com" in order to find any matches, and would proceed to use that file to execute whatever instructions you'd given it.
If DOS couldn't find your command in the "command.com" file, it would then search all executable files in the directory you were currently browsing for a match. The current directory could be changed by using the "DIR" command, which would display a list of all directories on a person's computer.
If DOS couldn't find an executable file that would carry out your command in the directory you were browsing, it would then look in any and all directories related to the environment path of the computer to search for a match for the user's command.
As computers at the time were extremely limited in capabilities compared to the complex machines of today, if your command could not be executed it was more than likely because you incorrectly typed it in, or were trying to execute a command that didn't exist. It would display that you had a "Syntax Error," which meant that the command could not be understood by the computer. You would then be free to try again.