In general, a persona is something which represents the identity of an individual or group. The term derives from the Latin word ‘personare’ that means to sound through. In ancient Greek and Roman theatre masks were used to denote characters in plays.
In our modern times, a persona can be described as a social role or identity assumed by an individual. It is also used to describe the personality of said individual and may further include behavioral traits, speech patterns, style of dress etc.
Personas are used in marketing to represent a target demographic. Marketers create personas from data collected through surveys, interviews and focus groups.
Personas are also used in social sciences. For example, a psychologist will create a persona of an average individual suffering from depression or anxiety and try to imagine how they feel.
However, personas are used more broadly than just marketing and psychology. They are used in fields like anthropology or sociology to represent a group of people in a society.
In the context of your question, you can think of audience personas as archetypes. They are not real people but rather fictional characters created to represent a group.