Karma Sannyasa is renouncing household or other duties at the fourth stage (avastha) of life in Hinduism. As per the teachings, it is renouncing the ego of being a doer (karta) and leaving the longing for the fruits of action.
Literally, karma sannyasa means renouncing the action itself, but as Bhagavan Sri Krishna says in the Bhagavad Gita, it is impossible even for a moment to remain without doing anything (Bhagavad Gita 3/5); if all activities are shunned, even life cannot continue (Bhagavad Gita 3/8). Therefore karma sannyasa does not mean sitting idle.
The real purport of the term is two- fold: of the four stages of ashrama dharma, the first three, viz., brahmacharya (celibate student), grihastha (householder) and vanaprastha (forest dweller) are duty bound, while the fourth, sannyasa (the renunciant), rises above all worldly duties and meditates on the absolute reality or the supreme being to realize the Brahman.
Even in the first three stage of life, one can renounce the false ego of being a doer. It is a recognized theory of the Hindu philosophical systems, namely Samkhya Yoga and Vedanta, that the atma or purusha (soul) is the ‘seer’ (drishta) and not the doer (karta). It is drishya or prakriti (primordial nature) which is responsible for all activities and gets all actions enacted through its three gunas (basic qualities), of course, for the purusha. The jiva (bound soul), out of ego, identifies itself as the doer of those actions because of avidya (illusive ignorance) (the Bhagavad Gita 3/27, Yoga Sutra 2/18-24. One who knows this truth and therefore has no ego (ahamkara) of being a doer and has neither enmity nor any longing for the consequences of actions is indeed a renouncer o action (karma sannyasi), even if seemingly involved in any action (Bhagavad Gita 5/3).
Sri Krishna says – he who gives up attachment to the fruits of action remain always content and does not depend on anything, does not verily do anything even though engaged in action. He does nothing even on having intense involvement (the Bhagavad Gita 4/20). Therefore, he who sees inaction (akarma) in action and action (karma) in inaction is wise among human beings (Bhagavad Gita 4/18).