What does the word "off" mean when it is describing someone's pronunciation?

As @YosefBaskin says in his comment "a bit off" means "slightly incorrect". One way to think of it is as being a short form of "slightly off target" or "a bit off target": "a bit" being an informal way of saying "a small amount".

In the feedback "off" indicates that there is a discrepancy between your pronunciation and the English pronunciation for which you are aiming but "a bit" indicates that the discrepancy is small.

"Off" can be used to mean any degree of error depending on the modifier used with it, for instance someone with quite poor pronunciation could be described as having pronunciation which was "a long way off". If someone else's pronunciation was really bad it could be described as being "miles off" or even "completely off" which again can be thought of as analogous to the accuracy of shooting at, or throwing something at, a target.

In terms of trying to hit a physical target "miles off target" is metaphorical (unless you are thinking of seriously inaccurate aerial bombing) but "completely off target" means that the shooter, thrower or bomb aimer has failed to hit the target at all. Think of a darts player whose dart has missed the board completely.