A command prompt is used in a text-based or "command-line" interface, such as a Unix terminal or a DOS shell. It is a symbol or series of characters at the beginning of a line that indicates the system is ready to receive input. It other words, it prompts the user for a command (hence the name).
The command prompt is often preceded by the current directory of the system the user is working with. For example, the default prompt in DOS may be C:\, which indicates the user is working at the root level of the main C: drive. In Unix, the prompt may be ~ user$, where "user" is the name of the current user. The tilde (~) indicates the current directory is the user's home folder.
A user can type commands at the command prompt, such as cd /, which means "change directory to the root folder." The "cd" command allows the user to browse through different directories of files on a hard disk or network. There are dozens of other commands that a user can type, which can be used to list, move, delete, and copy files, run programs, or perform other operations. While the "cd" command is the same in both DOS and Unix, many other commands have different syntax.
Since a command prompt requires specific input, it is basically useless if you don't know the syntax of the command you want to enter. Therefore, knowledge of some basic commands is required to use a command-line interface. Fortunately, most operating systems now use a graphical user interface GUI as their main way of requesting input from the user. This means you don't have to learn any special commands in order to use the computer. However, learning to use a command-line interface is kind of like learning to drive a car with a manual transmission. You never know when it might come in handy.