By default the top row of a Mac keyboard does what the icons on the keys say: brightness, Mission Control, media, volume. To get the function key, you hold fn. That is the reverse of a PC, where F5 reloads and fn is for the extras.

The switch is in System Settings → Keyboard → Keyboard Shortcuts → Function Keys: Use F1, F2, etc. keys as standard function keys. Log out and back in once for every app to pick it up. After that, fn plus a key gives the brightness or volume control.

Two things to know:

  • On a MacBook, F5 and F6 are keyboard-backlight keys; with the setting on, they are plain F5 and F6, which is what a browser expects for reload.
  • The fn key (labelled 🌐 on newer keyboards) also opens the emoji picker when tapped alone. That is a separate setting under Keyboard → "Press 🌐 key to", and can be turned off.