The gag reflex is a reflex contraction of the back of the throat that prevents objects from entering the throat except as part of normal swallowing. This helps prevent choking.
Touching the soft palate evokes a strong gag reflex in most people, although people can train themselves to resist the gag reflex, for example to be able to perform "deep throat" oral sex, or as part of the act of sword swallowing.
Absence of the gag reflex is a symptom of a number of severe medical conditions, one of which may be damage to the glossopharyngeal nerve.