Lord Subrahmanya's six faces represent the following

The Hero of the Skanda Purana is Lord Skanda Himself, known otherwise as Lord Subrahmanya. He is depicted with six faces and twelve hands. His Svarupa is Jyotirmaya. (Form of light, I suppose) On the Earth plane, every human being is an outcome of the five elements and the spirit within him. When the Lord took human form, He was accordingly depicted similarly. Hence the spirit, signified by the sparks from Siva, appeared as a human personality whose body comprised the five elements and who is the Lord of them. But to distinguish Him from ordinary human beings, the author thought it better to bestow six faces and twelve hands for the Divine-man.

These six faces represent the following:

(i) Five, one element each: Ether, air, fire, water and earth. The sixth face represents the Spirit that bestows LIFE to the beings formed by the five elements.

(ii) The Sanskrit word Bhagavan means one possessing the six godly attributes.

And each face of Lord Skanda personifies one of such functions or attributes of the Almighty.

(iii) In the Tamil scriptures, a clear account of the functions of the six faces of the Lord is given. One face is shining to remove ignorance and bestow Knowledge; another fulfils the desires of devotees and attends to their needs; a third face bestows strength and inspiration for the performance of rituals and sacrifices; a fourth brings out the inner secrets and helps the seekers of knowledge by revealing those secrets to them; a fifth face has the purpose of protecting the virtuous and punishing the wicked; and the sixth face stands for that function of the Lord that ignites Love and bestows happiness amongst the Jivas.