Keyboard accelerators are letters with an underscore that you see on buttons, labels etc. This feature is very useful for people who prefer to use the keyboard. For example, instead of using your mouse, you can click on OK by pressing Alt+O.
The downside is that it makes applications appear more cluttered. But not anymore. Starting from 4.8 (or did I just not notice this awesome feature until now?), you can configure Oxygen, the default style for KDE applications, to hide keyboard accelerators until you hold down the Alt key, or make them disappear completely.