The word 'Jagannath' is derived from two different words 'Jagat' and 'Natha'. 'Jagat' stands for the 'Universe' or the 'World' and 'Natha' implies 'Lord' or the 'Master'. Therefore, the name Jagannath literally stands for 'Lord of the Universe'. Puri is the home of Lord Jagannath and hence it is otherwise called 'Jagannath Dhaam' or 'Jagannath Puri'. Lord jagannath is the presiding deity of Sri Mandir or the Puri Jagannath Temple. Within this temple He is worshipped with his elder brother Lord Balabhadra, younger sister Goddess Subhadra and the all-powerful Sudarshan Chakra. Lord Jagannath is black in colour which signifies his inscrutability. We refer to someone as inscrutable when it is very difficult to understand him. Same is the character of the Lord who is the most mysterious in the whole of the Universe. Lord Balabhadra is white in colour which signifies enlightment. Goddess Subhadra is yellow in colour which signifies goodness and prosperity. Lord Sudarshan is red in color which signifies tremendous power. Together the four of them symbolise all the four runs or yugas of the world, that is to say, 'Satya-yuga', 'Treta-yuga', 'Dwapara-yuga' and 'Kali-yuga'. 'Badrinarayan' in Badrinath Dhaam was worshiped in Satya-yuga, 'Sri Ram Chandra' in Ramesvaram in Treta-yuga, 'Dwarakanath' in Dwaraka was worshiped in Dwapara-yuga and 'Sri Jagannath Mahaprabhu' in Puri is worshiped in Kali-yuga. At present we are living in the Kali Yuga and the supreme power in this yuga is none other than Lord Jagannath.
In Sanskrit which happens to be one of the most ancient languages, 'Ja' stands for Jagannath, 'Ga' stands for Balabhadra, 'Nna' stands for Subhadra and 'Tha' stands for Sudarshan. So Jagannath stands for the unity of all four deities of the Sri Mandir. His arms are parallel, thus ever ready to embrace the devotees and lovers from all over the world and his eyes are round and full without any eye-lids, thus taking care of the well-being of his devotees without sleeping for a moment. Everything concerned to Lord Jagannath is 'Bada' which stands for his immensity and enormousness. He is called 'Bada Thakura' or the head among all Gods. His temple is called 'Bada Deula' or the main temple among all temples. The road in front his temple is called 'Bada Danda' or the grandest among all roads. The Sea of Puri is called 'Mahadadhi' or the greatest among all oceans. In Puri there is a Hindu cremation ground called 'Swargadwar' or the gateway to heaven. There is a belief among Hindus that whosoever, after death, is cremated here finds 'mukti' or freedom from the cycle of birth and death.
Now why is Lord Jagannath black? As already said, the black colour signigies his inscrutability. The black colour also resembles the sky which is vast and mysterious. He is the Lord of the Universe, regarding whom the four Vedas are quiet. Oftentimes people are mystified about the enigma that circumvents the Lord. Some people believe that Lord Jagannath is a great void or a 'Maha Shoonya'. Yes, he is 'Maha Shoonya' or a great zero. You add zero with zero, the outcome is zero. You deduct zero from zero, the outcome is zero. You multiply zero with zero, the outcome is also zero. Zero is the state of nothingness and nullity. It is also the fullness and comprehensiveness. Zero symbolises the completion or culmination of a process. Another question rises in the mind that why Lord Jagannath does not have full grown hands and legs? Why is he a 'Madala' or someone who is unable to move? The answer is, the 'Jeeva' or the living being executes 'karmas' or duties and obligations but 'Param Brahma Parameshwar' does not and hence, he has no hands or legs and he does not execute any 'karma'. Therefore, he is static and immovable. Again, why are the eyes of Lord Jagannath round-shaped, unusually big and without eye-lids? He himself is 'Maha shoonya' and looks after the entire world through his eyes and never blinks them ever. In his eyes, all mortals are equal irrespective of their 'jaati' 'dharma' and 'varna'. Therefore, his eyes are called 'Chaka Dola' or eyes as big as wheels.