When you press and hold a key on the keyboard the previous default behavior was for it to repeat the key, now when you hold down a key it shows a character picker. You can change the behavior with this tweak.
Start a new Terminal session (in Finder click Applications > Utilities > Terminal).
To set this to Character Picker, type (or copy and paste) the following command:
defaults write NSGlobalDomain ApplePressAndHoldEnabled -boolean true
To set this to Key Repeat, type (or copy and paste) the following command:
defaults write NSGlobalDomain ApplePressAndHoldEnabled -boolean false
To reset this setting to the default value, type (or copy and paste) the following command:
defaults delete NSGlobalDomain ApplePressAndHoldEnabled
To activate this tweak please reboot the system.
Close the Terminal session by typing
exit at the prompt.
Disclaimer: Making any changes to your system can cause serious problems that may require you to reinstall your operating system. We cannot guarantee that problems resulting from these modifications can be resolved. Please follow the instructions carefully and use the information provided at your own risk.