Chiral fermions are spin-1/2 particles, such as quarks and electrons. They are either left- or right-handed depending on the orientation of their spin relative to their momentum. For the most part, the handedness state of a fermion stays constant, a property called chiral symmetry. Theories run into problems when they break chiral symmetry, which happens when the model changes a fermion with a left-handed state, for example, into one with a mixture of both chiral states.