Doctor Howe is Samuel Gridley Howe, founder of the Perkins Institute, the first school for the blind and visually impaired in the United States. Both Annie Sullivan and Helen Keller attended school there. Doctor Howe is famous for his revolutionary methods for teaching the blind, and reknowned especially for his work with Laura Bridgman, a girl who was both blind and deaf like Helen Keller - he taught her to communicate using sign language and raised print, pioneering the new field of deaf-blind education.
When Annie arrives at the Keller's, she has been studying the works of Dr. Howe. From his writings she understands that her job - "first, last, and in-between", is to teach Helen language. In answer to the second half of your question, the play does not mention that Dr. Howe actually told Annie anything directly, although she does remember that he said in his writings, "Language is to the mind more than the light is to the eye". Also, Annie does say that she has three advantages over Dr. Howe in being chosen to work with Helen - she has "his work behind (her)...hav(ing) read everything he wrote", she is young, and she has been blind herself (Act I).