The "a" in this sentence is known as the personal "a".
From StudySpanish.com: 'In Spanish, when the direct object is a person, it is preceded by the preposition "a." This word has no English translation.
From the perspective of the English speaker, the personal "a" appears to be an extra word. From the perspective of the Spanish speaker, the personal "a" is required, and to not use it is a serious error.'
See http://www.studyspanish.com/lessons/persa.htm for further explanation and more examples.