The bull Nandi is Shiva's vehicle and the main gana (follower) of Shiva. Nandi is Gate keeper of Shiva's abode: The close association of Shiva and Nandi explains the presence of a statue of Nandi at the gate of many temples dedicated to Shiva. In Sanskrit, a bull is called vrisha, which has another connotation — that of righteousness or Dharma. Therefore it is important to seek the blessings of Nandi before proceeding to worship Lord Shiva.In addition Nandi is Shiva's foremost disciple. In the Natha tradition, Nandi is one of the primal gurus. He was the guru to Siddhar Thirumulanathar, Patanjalinathar and others.
From the yogic perspective, Nandi is the mind dedicated to Lord Siva, the Absolute. In other words, to understand and absorb Light, "the experience and the wisdom" is Nandi which is the Guru within.
Spiritually, Nandi represents an individual jiva (soul) and the message that the jiva (Atman) should always be focused on the (Paramatman).
The four feet of Nandi represent forgiveness (Kshama), kindness (Daya), donation (Daan) and asceticism (Tapa). By worshipping Nandi one becomes focused, sensible, intelligent and knowledgeable.