Short answer
Open Terminal and run defaults delete com.apple.dock; killall Dock. The Dock restarts with Apple's default apps and settings. Back it up first with defaults export com.apple.dock ~/Desktop/dock-backup.plist if you might want your setup back.
Step 1: back up your current Dock
Resetting throws away your apps, folders, size and position settings. If there is any chance you want them back, save them first. This writes a copy of your Dock settings to your Desktop:
defaults export com.apple.dock ~/Desktop/dock-backup.plistStep 2: reset the Dock
Open Terminal (in Applications, then Utilities) and run:
defaults delete com.apple.dock; killall DockThe first part deletes your Dock preferences. The second restarts the Dock so it reads the defaults again. The screen flickers for a second while the Dock comes back. That is normal.
Step 3: restore your backup, if you change your mind
defaults import com.apple.dock ~/Desktop/dock-backup.plist; killall DockThings a reset does not fix
- The Dock keeps hiding or moving screens. That is usually in System Settings, then Desktop and Dock, not in the Dock preferences.
- An app icon shows a question mark. The app was moved or deleted. Drag the icon out of the Dock and add the app again.
- Folder stacks look wrong. Right click the stack to change how it sorts and displays.
If you reset because you keep rearranging it
Many people reset the Dock because it has turned into a mess from swapping apps in and out for different work. DockAlly keeps a separate Dock for each kind of work and switches between them, and it backs up your original Dock before its first change so one click puts it back.