This doesn't tell me anything about what jailbreak is
inb4letmegooglethatforyou

Jailbreaking an iDevice is basically modifying the system slightly and adding an app to allow people to heavily modify their device. Because iOS is Unix based, you'll see a lot of similarities between it and Linux when you get down into the nitty-gritty stuff.
Jailbreaking allows you to do things like replace the default multitasking system, change how your lock screen worls, or allow you to access all system files on the device.