This is one of the less known and hidden concepts of Vedic Astrology. At the same time, it is very important to understand this concept to understand why results in Astrology gets tweaked sometimes. First of all, let's understand what these 2 terms mean, i.e. Jeeva Karaka & Ajeeva Karaka. Jeeva means Soul and Karaka means Significator.
Jeeva Karaka means significator of those things which has soul or life.
Ajeeva Karaka means significator of those which has no soul or life.
So, every Planet or House represent a truckload of things. Like 1st house represents Self but it also represents East Direction & Place of Birth. 3rd house represents Younger Siblings & Neighbors but it also represents your Skills, Interests and Hobbies. Like this every House or Planet represent many a things.
Now, some of these things are alive, means they have a soul, hence they are Jeeva Karaka of that House or Planet. Rest of the things are not alive, means those don't have a soul, hence those are Ajeeva Karaka of that House or Planet.
Like, Lover or Children are Jeeva Karaka of 5th house but Education & Hobbies are Ajeeva Karaka of 5th house. Likewise, Spouse/Partner is Jeeva Karaka of 7th house but Business/Market Place are Ajeeva Karaka of 7th house. Same way, Husband is Jeeva Karaka of Jupiter for a Girl but Wealth is Ajeeva Karaka of Jupiter for all. So, whichever representation has Soul or Life involved is Jeeva Karaka and whichever has no Life/Soul involved is Ajeeva Karaka.
Now, why it happens that a natural malefic planet sitting in a House or with another Planet impacts some of the representations of that house or planet negatively but doesn't impact other representations as such?
It is for the reason of these two different types of Karakas. So, a malefic planet will impact Jeeva Karakas of a house or another planet negatively (if there is no other support to house or planet) but its impact on Ajeeva Karakas won't be so negative.
Like, if Saturn is in 5th house, person may not have smooth love relations or may have delay in child birth (if there is no other support) but he will be very focused on his studies because of which other things like Love Matters or Family take a back seat. Likewise, Rahu in 7th house may give a hard time in dealing with Marriage and Spouse, but person will get Fame among Masses if he pursues right Business. Mars in 3rd house may give stress, arguments and dominance struggle with siblings and neighbors but at the same time, it will give extreme courage to person.
Likewise, if Saturn is with Venus, it may give difficult initial relations and marriage may be delayed but it can make person very creative as person is very focused (Saturn) on Creative Arts (Venus). If Rahu is with Jupiter, it may make a person filthy rich but it gives hard time in accepting a Guru. Mars with Mercury can make an excellent Engineer but person may have a harsh/blunt speech towards others, which may spoil relations and friendships.
So, Jeeva Karaka are living significators of a planet or house, which are negatively impacted by a malefic planet's presence and Ajeeva Karaka are non-living significators of a planet or house, which are not so impacted by a malefic planet's presence.
Hope this helps. Thanks,