Phone hacking is the practice of exploring a mobile device often using computer exploits to analyze everything from the lowest memory and central processing unit levels up to the highest file system and process levels.
Modern open source tooling has become fairly sophisticated as to be able to "hook" into individual functions within any running App on an unlocked device and allow deep inspection and modification of their functions.
Phone hacking is a large branch of computer security that includes studying various situations exactly how attackers use security exploits to gain some level of access to a mobile device in a variety of situations and presumed access levels.