Retrograde is the time when the planet, as per astrology, moves in backward direction.
In retrograde position, Jupiter usually gives good results.
When in its own house, uccha sthana or strong position, its effect increases. Jupiter in uccha sthana becomes paramoccha in retrograde condition.
While if Jupiter is neecha and is retrograde, it gives some malefic effect related to that house. Usually, it is called 'danda' (punishment) related to that house.
On the other hand, retrograde Shani in neecha awastha becomes ucchha and after givng bad effects due to ill pplacement, it starts giving good results.
Not going by individual definitions by most of the astrologers are of the view that a retrograde Jupiter usually gives good results in good places while most of the astrologers differ in the statement about the effect it produces in bad positions. As per my knowledge and what I have seen in kundli of people, it definitely gives some bad effect like some people become adamant and headstong, some have life long diseases even though they are not severe but it also gives good effects being in retrograde condition like increasing wealth sources from that house, improvong metal ability etc. But if it is in neecha awastha and still retrograde, it usually gives bad effects.