Generally speaking, a string-like part of the root is exposed that part of the root is dead. In this case it's because of lack of humidity. I personally mist my exposed roots, to keep them healthy and prevent total dry-out. Orchids thrive on humidity as well. Hollow or mushy roots to the touch are considered dead as well and can be trimmed off. Roots that feel solid are generally the living roots.
It's important to remember when orchids are in there natural habitat they go through rainy and dry spells. Within the dry spell, it rains, the roots absorb as much nutrients and water as possible for survival through another dry spell...so rule of thumb, if you feel your orchid might need watering it will not hurt to wait a day or two. Orchids can survive on dry spells but have a hard time surviving when there is to much moisture.
“Respect does not come from the work you do, it comes from the way you do your work.”