You are trying to install or open an app that didn't come from the Mac App Store but you got a message saying App can't be opened because it is from an unidentified developer. This is how you bypass it.
If you want to make a clean install of the new OS X Mavericks or even the older versions of Mac OS X, than I strongly suggest you to create a bootable OS X key or a drive.
Apple Mac OS X Lion and Mountain Lion don't come with a button that enables the ftp server by only a click, but that still doesn't mean the ftp server isn't included. Let's see how to enable it.
Sometimes you don't want your Mac OS X Mountain Lion powered computer to go to sleep, here's a nice little trick how to temporary prevent a Mac from going into a sleep mode.