It is a common expression used to emphasize something that is true or to correct a previous statement.
For example:
* "Je pensais que tu étais en vacances, mais bi en fait, tu es au bureau." (I thought you were on vacation, but actually, you're at the office.)
* "Il a dit qu'il n'avait pas le temps, mais bi en fait, il ne voulait pas venir." (He said he didn't have time, but actually, he didn't want to come.)