Glossary
Personal data
Also: PII
Personal data is any information relating to an identified or identifiable natural person, under GDPR. It is what is loosely called 'personally identifiable information' (PII), though the GDPR term is broader: a name or email obviously, but also an IP address, a device id, or anything that can be linked back to an individual.
In code
The hard part in code is that personal data looks like any other field in a payload. A change can quietly start logging it, sending it to a new third party, or storing it with no retention, and nothing in the code flags that the data is regulated.