What is primary adrenal insufficiency (addisons disease)?

Adrenal cortex: The adrenal cortex makes lots of hormones. Very important are cortisol and aldosterone which help maintain life and salt balance. Addison's disease is caused by adrenal cortex damage rendering it unable to make these hormones anymore, mostly autoimmune cause in us. Patients often have a bronzed look. Hydrocortisone replacement is a lifesaving necessity. Fludrocortisone may also become necessary.

Often, yes: Addison's disease is an autoimmune disease which harms the cortex of the adrenal glands. However, anything that damages the adrenal cortex can also cause primary adrenal insufficiency.