In plain words
Niṣiddha karmas means: there are certain particular karmas we must give up. This is the chief duty we must carry out in the matter of food and recreation. One must examine oneself: "In what kind of conduct am I?" One must reflect, "Is my conduct true to the word I spoke and the thought I thought, or not?" But thinking, "No one knows my thought, surely," you may bring about changes in your words and deeds; the people of the world do not know your thoughts, but the Lord of the world knows them.
You can cover the eyes of the world, but Citragupta, who writes the ledger: his eyes, how will you blindfold them?
Wondrously hidden, God dwells in your citta. You may deceive the world; you may hide things from the world. You may blindfold the world's eyes. But you cannot blindfold the eyes of God. Sooner or later there is no escaping this fruit for us. The doctrine of karma and its fruit, at least, we must believe without fail. So our land of Bhārata is the very one that accepted the doctrine of karma and its fruit. That is why in the Bhāgavatam too:
By karma a creature is born, by karma alone it grows, by karma it dies; karma is God to man, karma is the cause of joy and sorrow in this world.
Karaṇaṃ, kāraṇaṃ, karma: the blending of these three is the life of man. So man must develop holy feelings within himself. To develop holy feelings, sāttvic food is most necessary. It must be both moderate and wholesome. Even when it is sāttvic food, we must not eat too much of it. We must not give our whole life over merely to the karmas of the world. Life is a long journey. On this long journey we must gain the strength we need for the goal we have to reach. So, for man to recognise his own reality by holy food and holy feelings, there are certain kinds of prohibitions. That is, there are some things we must give up. If we want to plough a piece of land and raise a crop, we must pull out the thorn bushes that were there before and all the useless growth.