Tab completion on linux
This video describes "tab completion," where the linux shell fills in details of a command or path or file that you are typing, when you hit the tab key.
In particular, if you hit the tab key while typing a command, the shell will try to complete the command or file/directory for you, given what it can figure out. If it is unambiguous what you mean, it will completely fill in the path. If there are alternatives, it will fill in as much as possible, and then if you hit tab again, it will show you the alternatives.