Where i can find variable?

It is the string that is your command prompt. For example, if you interactively start a PuTTY session, you login, and now the terminal window is waiting for you to interactively enter a command. It will display a prompt. It is whatever that prompt string might be. For example, when you interactively type a shell command, such as "ls" and press RETURN, the output of the command is displayed in the terminal window, followed by the next command prompt, where you may then type the next command.