Personas are a framework to create and bring archetypal customers and users to life. By focusing on discrete types of users, it makes it easier to visualize and imagine how these actors would interact with a system and identify important use cases and features. They have been in use since the mid-90s and are a common part of ethnographic research.
Personas are visual stories that describe actual customers. Personas draw a detailed picture of the user, what they want to accomplish, what knowledge or skills they may or may not have, and why certain things may be more important to them. The products or services you design try to solve these customer problems. Personas help the team focus on user-centered design.