In Hindu tradition, many devotees apply the holy ash known as Bhasma or Vibhuti on the forehead. The bhasma is the holy ash obtained after the wood is burnt in the homa. Here a special wood is used and other materials like ghee and other herbs are offered into the sacrificial fire.
The word Bhasma is derived from Bha meaning bhartsanam (to destroy) and sma meaning smaranam (to remember). Bhasma means that by which our sins are destroyed and the Lord is remembered. Applying Bhasma symbolises the destruction of the evil and remembering the God. Bhasma is also known as Vibhuti meaning glory .
Hindu devotees of Lord Shiva are especially known to apply bhasma or vibhuti on the forehead. Some devotees also apply the bhasma on certain body parts like the chest and upper arms.
The bhasma is sometimes used for worship of Lord Shiva and poured all over the shivling and then distributed as prasad among the devotees. People also consume a pinch of the holy ash offered as prasad. It is believed to protect the devotee from ill health and evil influences.
Bhasma is also known to have many medicinal value and is used in many ayurvedic medicines. It is known to absorb excess moisture from the body and prevents cold and headaches. This is due to the combination of the various herbs and ghee used during the homa.
Also the mantras chanted during the homa would add special powers to the holy ash and hence it is applied by the devotees.