The extra eye represents the wisdom and insight that Shiva has. It is also believed to be the source of his untamed energy. The vibhuti are three lines drawn horizontally across the forehead in white ash. They represent Shiva's all-pervading nature, his superhuman power and wealth. Also, they cover up his powerful third eye. Followers of Shiva often draw vibhuti lines across their forehead.
All You Need to Know About Hinduism: Hindu God "Shiva"