What is the fn key for on a Mac?

The bottom-left key on a Mac keyboard, marked fn or with a globe, has three jobs:

• The top row. Out of the box the keys F1–F12 are brightness, Mission Control, media and volume; holding fn turns them into F1–F12. System Settings → Keyboard → Keyboard Shortcuts → Function Keys flips that, so they are F-keys and fn gives the media functions.
• The missing keys. On a MacBook, fn← is Home, fn→ End, fn↑ Page Up, fn↓ Page Down, and fn⌫ is the forward Delete.
• A tap on its own. By default it opens the emoji picker; System Settings → Keyboard → "Press 🌐 key to" can make it change the input language, start dictation, or do nothing.

A PC keyboard has no fn key and does not need one: its Home, End, Page keys, Delete and F-keys are real keys, and a Mac reads them as such. Insert does nothing on a Mac.

Some Apple keyboards also use fn for a few shortcuts: fnF is full screen, fnD dictation, fnE the emoji picker, fnQ a quick note.