/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; keys are shown in uppercase as on a keyboard

File, File > Save As: menu items, buttons

Dancing Penguins (Chapter Penguins, ↑Another Manual): This is a reference to a chapter in another manual.