Makara is the name of a Zodiac sign in Indian astrology (Capricorn in English). "Jyothi" means "light" in Sanskrit. Thus "Makara Jyoti" means "Light of Capricorn".
In the sky, we have 12 Rasis / Zodiac signs or constellations. The Sun appears to move from one rasi / zodiac constellation to another every month and the day on which Sun changes the constellation is called Sankranti. Like this the sun covers all the 12 Rasis in a year.
The sun is constantly moving between the Tropic of Cancer and Tropic of Capricon which is called in the Indian context as Uttarayana and Dakshinayana. Makara Sankranti falls in the period when the Sun moves from Tropic of Capricorn (Dakshinayana) to Tropic of Cancer (Uttarayana) in India.
Makara Jyothi is a celestial star which is worshiped by pilgrims in huge numbers at Sabarimala Ayyappa Temple in Kerala on Makara Sankranti on January 14th or 15th every year. It is a bright star that appears and glow on the horizon above the holy hillock of the Sabarimala Sannidhanam on Makaravilakku day (at the moment of Makarasmkarama pooja).
Makara Jyoti (Star) is not to be mistaken to Makara Vilakku (Holy flame). Makaravilakku is a traditional symbolic light or flame (Deeparadhana / Arati offered to the Deity) that appears thrice on the Ponnambalamedu hill, some kilometer away to the Sabarimala temple. In other words, Makarajyothi is not man-made. Devotees believe that the Makarajyothi darshan brings them prosperity and divine blessings. It is also believed that on Makara Vilakku day, Lord Dharmasasta (Ayyappa) stops his tapas to bless his devotees.
Thiruvabharanam is the sacred gold ornaments of Lord Ayyappa. Every year during Makara Vilakku ceremony, the ornaments are taken to Sabarimala as a procession and poojas are performed on the idol adorned with the ornaments.
The very name Ayyappa in the South Indian languages, denotes the embodiment of 5, the 5 Primordial elements (Space, Air, Fire, Water & Earth) of which this Universe or Prapancha is made of.
Celebrating the festival of this Makara Sankranti in association with Lord Ayyappa, brings to our attention that we and everything in this Prapancham is made up of Panchabhuta - the Primordial five elements.
When we say, Ayyappa Saranam, we pay obeisance to these 5 Primordial elements themselves. We pay obeisance to Ayyappa, son of Shiva the cause for the manifestation of matter and Vishnu, the all pervading force which together keep this Universe conforming to the Dharma of the Universe and Nature.