When the object of a verb (except tener) is a definite person, it is preceded by a. It isn't used if a
number precedes the object though. The pronouns alguien (somebody), alguno (someone), nadie
(nobody), and ninguno (no one) require a as well, when used as the direct object. It is also used
before geographical names that are not already preceded by an article.
Veo a Juan. I see John. Veo a alguien. I see somebody. Describe a Chile. Describe Chile.