Traditionally rings a bell.
Current implementations vary in how this is interpreted. Most implementations still support an audible signal but often also offer setting window manager urgency hints or other advanced reactions.
🛈 audible beep, deiconify, urgency hint, visual bell
🛈 audible beep, urgency hint, visual bell, raise window
🛈 audible beep, terminals that use vte can add additional behavior.
🛈 audible beep, visual bell, KNotify