/etc/passwd: filenames and directory names

placeholder: replace placeholder with the actual value

PATH: the environment variable PATH

ls, --help: commands, options, and parameters

user: users or groups

Alt, Alt-F1: a key to press or a key combination

‘File’, ‘File’+‘Save As’: menu items, buttons

Dancing Penguins (Chapter Penguins, ↑Reference): This is a reference to a chapter in another book.