Simplified it is the data relating to a physical person who with this data can be identified directly or indirectly.
The GDPR definition of personal data is stated in Art. 4(1) GDPR as:
“Any information relating to an identified or identifiable physical person (‘data subject’) (i.e. not a legal entity); an identifiable physical person is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person.”