Hereth illint is an Andalite term roughly translating to "burping DNA".
Because Andalites do not have mouths, they do not have words or phrases for "spitting up" or "vomiting out." Hereth is the nearest equivalent.
When an individual acquires DNA to which they are adverse or allergic, they can lose some control of the morphing ability. This loss of control can result in spontaneous morphing and rapid morphing without transitioning through the individual's base form. The loss of control will continue until the offending DNA is expelled.
According to Ax, being allergic to DNA and performing a hereth illint are not common but not unheard of. However, the only known case of a DNA allergy among the Animorphs occured when Rachel acquired a crocodile.