can anybody explain me what does the sentence mean "Apparently, this user prefers to keep an air of mystery about them."
From MW Learner's Dictionary,
air of mystery: a mysterious quality
The statement you have given above means to convey the following:
"It appears that this user likes to be mysterious. The user may not wish to disclose details regarding himself/herself and thus prefers to be mysterious. It could mean that there is a lot more to the user than meets the eye, but he/she prefers to be quiet about it. At the same time, it is sometimes intended to generate curiosity and intrigue about the person.