If my 6 year old complains of a headache what could that be? Or is it just a headace.

Depends. Kids get headaches (ha) from illness, trauma, stress, migraines, and more. Identifying a cause is sometimes easy (such as with illness- which might be treatable, like strep) but often a ha diary of any possible triggers is helpful. Include amount of sleep, foods/drinks, sunlight, noise, smells, and anything else you can think of. Diary days even with no ha to compare. Doc can then help sort out.
Headaches. Some children (boys more than girls) develop migraines at this early age, especially those with a family history of migraines. Other causes could be sinus problems, eye strain, and other more serious medical problems. See your doctor for evaluation.
Maybe... Depends upon the nature of the complaint; and any other factors(trauma, illness, stress, etc.) so please monitor your child's symptoms and take him/her to the pediatrician for an evaluation! if the symptoms persist.
6 y/o is a concern. A 6year old complaining of headaches is a concern. Start with your pediatrician, who may refer you to a pediatric neurologist.