In plain words
The sorrows that take hold of man in the world are three: ādhyātmika sorrow, ādhibhautika sorrow and ādhidaivika sorrow.
Ādhyātmika sorrow means suffering from disorder of the Manas and from diseases; the human body is a house of filth, the dwelling place of worms, germs, insects and the like. Sickness of the mind comes about only through bad qualities. If you foster the sacred feeling that is compassion, you have the chance to ward off ādhyātmika sorrow. However clever a man may be, unless he is a man of virtue, he is not worthy even of being human. Virtue itself is the chief splendour. If you bring out good qualities and make them shine, you can gain Ānanda too along with health; it is bad qualities that are the root cause of disease. Through bad qualities, old age too sets in very quickly. Through good qualities, lasting vigour, strength and ability, youthfulness, all of these grow.
Right in your student days, you must not let bad qualities, bad thoughts and bad habits come near you, and you must cultivate sacred good qualities. Through Ānanda, health is won. A healthy heart is the home of Ānanda. Young men must make themselves uplifters of the world. They must fill their hearts with good qualities. Measured speech and measured food: these you must practise in your daily life at this young age itself. 'Jaise an, aise man,' they say. You must set yourselves to eat only sāttvika food. As the food is, so is the feeling; as the feeling is, so is the Buddhi; as the Buddhi, so the attainment: to gain the great treasure that is the presence of God, you must purify your food and your ways.
Ādhibhautika sorrow means being tormented by living beings, that is, the sufferings that come from germs, insects and the like, and from cattle, birds, beasts and the like. To ward these off, you must follow the path of yoga. Yoga means the restraint of the movements of the citta. It is that which curbs the movements of the citta. Then there is no suffering at all; whoever may revile you and whoever may praise you, Śānti of mind is the one thing you must not lose. When mastery of the senses is gained, Śānti of mind is won. For this, the sādhana of yoga is needed. We are arranging for you to be taught the āsanas of yoga. Through that your health will become excellent; you will gain the strength to ward off suffering, too.
To rid yourself of ādhidaivika sorrow, you must attain the state of samādhi. That sorrow means too much rain, no rain and the like, which come from God. Because of these, fear arises in man's mind, and he suffers greatly. Samādhi is not these: falling down, flinging your arms and legs about lost in rapture, speaking indistinctly. Dhī means the Buddhi; sama means evenly. Holding the Buddhi even, making no distinctions, neither sinking nor swelling: that itself is true samādhi.