How well does humira (adalimumab) actually work for psoriasis? And what does it mean to be moderate/severe?

Well enough. Humira (adalimumab) targets the tumor necrosis factor (tnf) which is involved in inflammation and immune response. However when there is too much, the target may get damaged. Psoriasis involving mainly the skin, phototherapy may be helpful. But for arthritis, Humira (adalimumab) (and other tnf inhibitors) is a good option. They all have similar side effects- the risk of severe infection. Good luck to you.
It works. There are risks to it though since it reduces your ability to fight infections. It does work though, but is quite expensive.