In plain words
The meaning of 'karma' commonly known to all of us is 'work'. Whatever dealing or business one undertakes may be called karma. In karma there is no high or low; every karma must be viewed with a spirit of reverence and practiced. There are many verses in praise of karma. It also happens that some Hindus commonly say 'karma, karma, karma', leaving everything to karma and prārabdha, and become lazy. This is an absent-minded linking: it has come about by tying the doctrine of karma together with worldly karma. Human beings do not know for what reason, or to whom, such warnings and counsels are addressed. Even so, ordinarily, look at those within our own experience -- a clerk who works in an office, a farmer who ploughs the land and lives by it, a labourer who lives by bodily toil, the potter, the blacksmith, the carpenter, the washerman, the barber and the like -- all of these know their own ordained duties. They know that if each does not perform their own karma, their life will not proceed along the proper path. So, on the very occasion when they are fulfilling their own respective karmas, according to their own respective strengths and abilities, what use are warnings? It is only at the time when they are unable to fulfill their own respective karmas that warnings become essential.
Being blinded by delusion, and unable to decide what his own duty was, Arjuna, in a spirit of dejection, said 'I will not wage war' -- and to him the Lord Śrī Kṛṣṇa said, 'karmaṇy evādhikāras te'. That is, some people also cite these nectar-like words of teaching, which taught that the performing of karma alone is one's duty, and this keeps happening. These teachings were given merely for the sake of establishing Dharma, and merely for maintaining engagement in the scriptural path of karma; they were not made in relation to the common, animal-like drives of food and fear and the like. Dharmarāja and the others were masters of worldly affairs, and were devoted to the observance of varṇa and āśrama dharma -- so what could be the purport of this beneficial counsel that tells such people to 'wage war'?