A Head Quarters (or simply a HQ) is the main building of the match. It doesn't require to be build as it is already built in the beginning of the match. The HQ can only train worker units. It can research Population Limit+ (to increase the population limit) and can also research advanced units, stealth detection, and other abilities. You should order several units to guard your HQ because your HQ is also the most important bulding of the match. If it gets destroyed it can extremely hard to win the match, and it cannot be rebuilt.