It is a document that describes a fictional character representing a segment of your target audience.
A good persona contains things like a picture of the character, its first and last names, its job, age, computer literacy level, a slogan that summarises well his state of mind, his motivations, a brief bio and all the traits of its segment of the target audience.
Apart from these “identity” pieces of information, it’s also useful to include the ideal scenario of this user interacting on your online project to fulfill a given set of tasks