On the bottom of the screen is a desktop panel called the "kicker". You use the kicker to launch applications, switch desktops and move between running applications & windows.
The button with a large "K" is called the Application Starter. It has a small arrow on top to indicate it will pop-up a menu if you click on it.
The pop-up offers easy access to most applications installed on your Linux system.
Shortcut: Alt-F1 opens the Application Starter menu.