A Halo in religious art is a symbolic representation of the aura or nimbus of light believed in many cultures to surround a Godly or enlightened person. Halos are used especially in Christian, Buddhist, Islamic, and Hindu religious images to depict holiness. Halos can also be found in ancient Roman and Greek art, which were direct influences on the practice in Christian art.
The practice is more than likely a holdover from the identification of deities with the sun.
Hindu Krishna; Apollo with halo; pre-Christian Buddha