Arbitrary read-write is an exploit capability that lets an attacker read data from and modify memory locations they can choose. “Arbitrary” means the access is not limited to a single fixed address or value; the attacker may be able to inspect sensitive data or alter program state, depending on the scope of the flaw.
This capability can enable serious follow-on attacks, such as changing security checks or helping execute code. It is often a prerequisite rather than a complete attack: another vulnerability or foothold may be needed to gain the memory access. In the Apple Pointer Authentication context, arbitrary read-write is the stated prerequisite for bypassing that protection, not proof that the bypass alone provides initial access or delivers malware. Defenders should patch affected software and investigate the earlier steps that could have provided the memory access.