The coconut represents the Hindu trinity of Brahma (the creator), Vishnu (the protector) and Shiva (the destroyer).
Devotees pay tribute to the three Gods by treating the coconut as an object of worship. Thus, they seek the blessings of the trinity.
The three dots in the coconut symbolises the three eyes of Lord Shiva. The kernel (white flesh) symbolises Devi Parvati, the water signifies Ganga, and the brown shell represents Lord Kartikeya.
The coconut is also compared to a human head. The fibre is the hair, the shell is the skull, the water is blood, and the flesh represents the brain. Therefore, by offering a coconut, a devotee surrenders himself/herself or the mind and bows before the Supreme Power.