One day a saintly person was standing in the river waters and saying his morning prayers. suddenly a crocodile grabbed his feet . The saint was nearly swept away and he prayed for his life to Lord Vishnu.
Lord Vishnu appeared in his Chaturvuj (four hands) form and using the Sudarsan chakra (the spinning knife) killed the crocodile. As soon as the corcodile died, his soul assimilated with Vishnu and was lifted to Baikuntha (Heaven).
The saint said: Lord it is I who prayed to you. It is I called out your name. But I am still standing here and it is the lowly crocodile who received salvation. Waht is the meaning of this?
Lord Vishnu said: Son, you are a saint. But the crocodile is a lost soul. He came and grabbed your feet, however crudely he may have done it. Any lost soul who falls on the feet of a saint, deserves salvation. You on the other hand must remain on Earth and continue to save souls. Because you are a saint and that is your duty or Karma. Saying this Lord Vishnu disappeared.
The philosopher King Krishna advised the great warrior Arjuna in the battlefield of Kurukcehtra thus: Look Arjuna, at the clouds above us. Those cluds will soon become rain and there shall be clouds no more. The rain shall become a river and there shall be rain no more. The river shall disappear into the sea and there shall be river no more. The sea will become a cloud again and there shall be a sea no more.
We must do our duty or our Karma. A soldier's duty is to defend the land no matter what the consequences. so you must defend the land.
Just as the Earth must turn every night to bring the day, just as the Sun must come up every morning to make life possible on the Earth, so must each man perform his duty without looking for its rewards.
This the message of the Gita and the meaning of Karma yoga.