While I refer to jailbreaking, there are a number of terms used out there to describe the same thing. In the Android world, rooting seems to be what is more commonly known. However, the premise remains the same. Rooting/Jailbreaking your device means that you escape the OS implemented sandboxing and gain full root access to your device. Many mobile applications are against this as a compromised sandbox may have occurred unknowingly, effectively meaning that the device is compromised. So, as a safety measure, applications check for this and refuse to run because of it.