The adrenal glands are two small, very powerful, glands that sit atop the kidneys. They are roughly 30mm wide, 10mm thick, 50mm long, and weigh around 5.0g. In customary measures that's roughly 1"x"2"x0.5" and 0.20/oz. They have multiple functions, from secreting hormones like epinepherine/norepinepherine (adrenaline), sex hormones, helping to regulate blood pressure and water retention by secreting other hormones like aldosterone and mineral/glucocorticoids, and helping to heal tissue and keep the body healthy by releasing cortisol (a type of glucocorticoid).
An analogy I like to use regarding the adrenals is like that of gas in the tank of your car. When we are born we have a full tank of fuel and a full reserve. As we live our lives we start burning some fuel, just like when we drive we use some fuel. When we get low on gas we stop at the gas station and fill up. Sometimes we get a little below the "E Line" on our fuel gauge before we stop and we tap into our "fuel reserve." Imagine that every time we tap into our fuel reserve we can't fill the fuel reserve back to 100%. If we dip below the "E Line" too often, eventually we will be extremely low on our fuel reserve part of the tank, since we can't fill it back to 100%.
This is similar to what happens to our adrenals with regard to cortisol secretion and chronic stress (which can be physical, emotional, and even environmental). After a while the adrenals may become fatigued or exhausted, we can't fill up the reserve any more and it's at zero, and we just don't produce enough cortisol any more. Not because we don't want to, but because we can't. I call this adreno-cortical hypofunction or adrenal fatigue (the CNS is directly linked to everything in the body and controls it). "So what?", you might ask. Well there are some glaring symptoms that can be due to adrenal fatigue, and here they are: