Reimpose jail time means to reinstate it, or send you back to jail. From what you write, you (or whomever you are writing about) were on probation from a prior offense, but there was apparently the option for jail time; e.g. you may have been granted probation on the grounds you do not commit another violation. Apparently you did commit such a violation, so the probation--the being out of jail, but subject to supervision--is revoked and you will serve out the sentence you would have had to serve the first time, minus or less any time already served (that's what "credit time served" means). Probation is often contingent on "keeping your nose clean" and not committing any other offenses.