Mac OS X installed something named "Script Runner". What is it?

Script Runner is part of the default install of Mac OS X.

The Official AppleScript site explains:

Located in the AppleScript folder, the Script Runner application is used to organize and execute AppleScript scripts which have been saved as compiled script files. When launched, it displays a small floating palette which is available no matter which other application is the current or frontmost application.

When clicked, the Script Runner palette will display a heirarchical menu of the Scripts found in the Scripts folder which is located in: Home > Library > Scripts. Selecting a script name from this menu will load and execute the related script file.

Since OS X 10.2 (aka “Jaguar”), the substitute is the Script Menu, a menu extra, similar to the old OSA Menu.