Becoming fairykind means that the Fairy Queen's and her fairies have infused a person with their magic. Ordinarily, a mortal must drink magical milk to perceive magic, though the effects wear off after a nights rest. After becoming fairykind, or a handmaiden of the Fairy Queen, a person will exhibit these magical properties indefinitely.
Once fairykind, a person is immune to magical forms of mind control such as distractor spells, can understand the Silvian language and other related to it (such as Gnomish, can see in the dark, etc.) A portion of the magical energy that naturally dwells in fairies now dwells within the person, and he or she can recharge magical objects and transfer some of that energy to fairies via physical touch. This person can also command fairies in the name of the queen to offer help or divulge information.
Being fairykind is a rare occurrence, even more so than being "fairystruck", to the point where it's all but unheard of. Kendra Sorenson is the only known person alive today who is fairykind. The Sphinx is the one who tells Kendra the truth about her being fairykind, and not fairystuck.