Long-pressing a key in macOS allows you to select a variation of the letter.
For example, long pressing 'e' shows this:
To repeat the pressed character instead of showing this, run this in the terminal:
$ defaults write NSGlobalDomain ApplePressAndHoldEnabled -bool false
Note: restart open applications to start getting repeating keys.
To show alternate characters again, run this in the terminal:
defaults write NSGlobalDomain ApplePressAndHoldEnabled -bool true