Selecting this command initiates the rendering operation.  Depending upon the state of the Allow Backgrounding parameter, this operation can be aborted by pressing (⌘-.).  Also the current state of the image can be copied to the window by clicking the mouse anywhere in the window if backgrounding is allowed.

Note that while a render is active, the Render menu item will always be highlighted.  This is one way to determine if the program is busy during a render.  Also, if backgrounding is on, the Render menu is the only program menu left active although the Render command itself is obviously not.