Murugar and Vinayakar are the two Sons of Shiva, who is the abode of the Yogas (the stillness/steadiness of the mind).  Murugar (Karthikeyar) is the bearer and dispenser of Gnana Pazham (or the Fruit of Knowledge), which is Knowledge of the Self.

Ganesha obtains a divine fruit of knowledge from Narada, after winning a contest with brother Murugar. While Murugar speeds around the world thrice to win the contest for the fruit, Ganesha circumambulates Shiva and Parvati thrice as an equivalent, and is given the fruit. After winning it, he offers to give the fruit to his upset brother.

Prayers invoking Murugar, invoke his protective powers and his ability to dispense the fruit of knowledge to His devotees.  .

(The following is an extract from Wikipedia)

Kartikeyar symbols are based on the weapons – Vel, the Divine Spear or Lance that he carries and his mount the peacock. He is sometimes depicted with many weapons including: a sword, a javelin, a mace, a discus and a bow although more usually he is depicted wielding a sakti or spear. This symbolizes his purification of human ills. His javelin is used to symbolize his far reaching protection, his discus symbolizes his knowledge of the truth, his mace represents his strength and his bow shows his ability to defeat all ills. His peacock mount symbolizes his destruction of the ego.

His six heads represent the six siddhis bestowed upon yogis over the course of their spiritual development. This corresponds to his role as the bestower of siddhis.