In relation to GDPR, Personal Data means any information that relates to an identifiable person (or data subject). Identifiable means a person who can be identified, directly or indirectly by an identifier such as:
Or other more physical relating factors such as:
Personal Data is a frequent and important subject in the GDPR and so it is important you know which data you have falls under which parts of the regulations.
For example, with Data Portability, you will need to know what data needs to be identified, packaged and then sent to the individual/new controller and then deleted.