In macOS, when a key is held down while entering text, a popup is shown which lets one choose between various accented forms of the character. To disable this execute the following command line in the Terminal :
defaults write -g ApplePressAndHoldEnabled -bool false
You’ll need to log out and log back in again for the setting to take effect.
This should disable the display of the popup and character typed should start repeating when the key is held down.
If you ever wish to revert to the orignial behaviour, execute the following command line in the Terminal :
defaults write -g ApplePressAndHoldEnabled -bool true
You’ll need to log out and log back in again for the setting to take effect.