In plain words
This Makara Saṅkramaṇa holds many kinds of meaning: physical, moral, of Dharma, spiritual and divine. In this Saṅkramaṇa the sun travels from the south to the north.
Uttarāyaṇa means turning the Buddhi inward!
What is meant by the northern direction? The place that grants Mokṣa is called the northern direction. Hima + acalaṃ = Himācalaṃ. What is this Himācala? Hima means what is pure. Hima means perfect whiteness. Acala means what is firm and unmoving. This pure snow, this tranquil snow, takes on an unmoving form. What is that? That place. It is the place of the heart. When the heart is in a state that is pure, unmoving and tranquil, this sun called the Buddhi enters into it fully. We need not make any outward, physical journeys somewhere. The journey of Uttarāyaṇa means turning our Buddhi inward. It means sending it in the direction of the heart; this inward joy is man's true joy. Outward, worldly, natural joys are not steady. They are only momentary. Eternal, true, pure, it is the Ātmatattva, and it remains without end, imperishable, deathless and infinite. This is what was called the abode of Mokṣa. The journey made towards it was called the means to Mokṣa.
Peace of mind and body comes on the festival of Sankrānti
Sankrānti means Saṅkramaṇa: the means that teaches the inward vision, the pure heart and the holy principle. Just as the sun travels towards the north, in the same way you must turn the Buddhi towards the Ātma and make it travel in that direction alone. The Veda has always taught that this time of Uttarāyaṇa is a sacred time. For this sacred day Bhīṣma lay waiting on his bed of arrows for fifty-six days. The teaching of our Purāṇas is that everyone who dies in this sacred time attains divinity.