KRunner is a versatile mini command-line. You can use it to start applications, open web pages, access bookmarks, search through your desktop data, calculate short equations, and many more. Pressing Alt+Space opens the KRunner dialog. You just start typing and KRunner will start searching matches as soon as you've entered more than two characters. You can open the settings dialog to learn about KRunner’s functionality, provided by plugins. You can navigate through the matches using the tab and arrow keys.

KRunner dialog

Use the button to open the Plasma Search System Settings module and configure where and what to search for in KRunner

If you want to know what is going on on your system, pressing Ctrl+Esc gives you quick access to a list of windows and processes, with options to monitor their output and kill processes.