If a particular iPhone has just been jailbroken and the jailbreak method is a tethered type jailbreak, then you will require this app. Use it only if you have previously created jailbreak files. There has been a lot of confusion between tethered and untethered jailbreak. Let me answer this in one line – an untethered jailbreak is always better than the tethered one.
The jailbreak files have IPSW file extension. You can download the Tethered Boot from here.
Here is the concept behind a tethered jailbreak – if you iPhone reboots or if the battery inside your iPhone peters out, you will have to connect it back to the computer and make it boot again with the help of jailbreak utility.