The verb "haber" in Spanish is primarily used to indicate existence or presence. It translates to "there is" or "there are" in English. For example, "Hay un libro en la mesa" means "There is a book on the table." This usage helps to express the existence of objects, people, or situations in a specific context. In addition to its existence function, "haber" is also used as an auxiliary verb in compound tenses. For instance, in the phrase "He comido," meaning "I have eaten," "haber" helps form the present perfect tense. This dual role makes "haber" an essential verb in the Spanish language.