As of 2015, using Cydia and JailbreakMe is legal for the iPhone and iTouch, according to Cult of Mac. However, after a congressional ruling, it is illegal to jailbreak your iPad in the United States if you bought it after Jan. 26, 2013.
"Jailbreaking" your iOS makes it possible to install hacks and modifications on your Apple product that the brand's devices normally do not accept, explains Cult of Mac. Cydia allows you to install this software. Users often confuse jailbreaking with "unlocking." Unlocking your iPhone makes it available for usage on a cell phone service other than the one through which you bought it. Unlocking a phone on your own is illegal.