"Karma" in its simplest term is "Action" only. Any action that we do with our body & mind is termed as "Karma" but the word "Karma" has a much greater significance as per the Hindu Philosophy than just simple meaning as "Action". In Hindu scriptures Karma is related to the origin of one's own body. Karmas of earlier birth become the basis of this birth & Karmas of this birth become the basis of his next birth & so on. This relation of Karmas to rebirth in Hindu Philosophy is the key thing which makes it different from doing just any action in life. Now how these Karmas are related to rebirth as per the Hindu Philosophy?
As per the Hindu Philosophy, any action that we do, produces some fruit or has a consequence. In simplest term, if a man works for 8 hrs in a mall, he expects his salary of 8 hrs at the end of his job. It may be given to him at the end of the job on that day, may accrue to his account weekly or monthly, but the man expects that he shall get his salary after doing the asked for job. So this "Expected Salary" is the "Fruit" of his 8 hrs of "Actions". Similarly any other "Action" or "Karma" done in life automatically has some "Fruit" attached to it. In terms of "Karmas" done by a man, all the consequential "Fruits" are determined by God himself. Say you donate some money to a Charity or you feed a poor man, what "Fruit" shall incur to you for this good work done by you? Or say some man beats a poor man or a man murders someone, then what "Fruit" shall incur to him for this ghastly act done by that person? In all such acts or Karmas done by all persons, the consequential "Fruits" are determined by God himself as per the divine laws & not by any human being.
So by now we have seen that all "Karmas" or actions done by a man have a Fruit related to it & that Fruit is determined by God only. But how these Karmas become the basis of Rebirth as per the Hindu Scriptures?
As per the Hindu Philosophy, any "Karma" done by a person, remains with that person, unless he/she has enjoyed the consequential Fruit arisen out of doing that Karma. For a good act done by him, he shall bear good result & enjoy it. Similarly for a bad act done by him, he shall bear bad result & shall have to bear punishment for it. Unless that person enjoys good or bad result/fruit arisen out of doing that Karma, it shall remain forever with that person & leave that person after that person has enjoyed the fruit arisen out of doing that Karma. Now whatever action/karma one performs with his body, mind or even speech, that Karma after getting finished, goes in to his reserved stock of Karmas i.e. it adds up to all the other accumulated karmas earlier done by him.
This stock of Karmas (having its reservoir in that person's mind) is having all the Karmas done by that person in this life & also done by that person in earlier births taken by that person. At the death of a person he/she takes with him/her all that accumulated Karmas which contain Karmas done in this birth & also the Karmas from the previous births for which its fruit is yet to be enjoyed .
So at the death of a person, out of this accumulated Karmas which that person carries, Divinity or God as per the divine law takes a handful of those Karmas which have begun to bear the fruits & a new body is made with the seeds of these fruit bearing Karmas. So basically any Karma done becomes the seed of the next birth. In Hindu Philosophy all these Karmas which become the basis of or the seeds of a new birth of a body, are called "Prarabdha" or "Fruit bearing actions" or "Fate" even. In the next birth, a person again does Karmas & these gets added to his earlier accumulated, unspent stock & so on this cycle goes on for a soul of taking birth after birth.
So in this way, so long as the reserve stock of Karmas remains with that person, a part of it continues to be taken out of that reserve stock by the Divinity as "Prarabdha" or "Fruit bearing actions", for being enjoyed in one life-time of that person. A soul's perpetual journey from one birth to next birth goes on like that with the law of karma. This soul knows no liberation, so long as this stock of un-disposed off actions or karmas performed in number of lives of that person is not completely exhausted.
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