Personal data is anything that can identify a person. An obvious case is a name, email, phone number, or address; a subtler one is data that identifies someone when combined, like a job title plus a company plus a city. Under GDPR it gets special protection.
For AI, the rule of thumb is simple: if a line would identify a real person, treat it as personal data and think before you paste it into a tool. When you only need the shape of the data, not the people, strip the identifiers first (anonymisation).