It's a little old-fashioned to refer to anybody as the "father" of anything. Patriarchy chafes in this day and age, (see Virginia Woolf's A Room of One's Own for a meditation on the unsung contributions of the foremothers), and assigning Chaucer such a grand title suggests that he singlehandedly "spawned" all of the rest of English literature. (He himself might laugh at the designation.) If we want to be careful in our use of language, it might be better to say that Chaucer made an important contribution to the far larger stream of English literature, a stream to which many were contributing in many different ways.
That being said, Chaucer did make important contributions. He might be seen in some circles as a "father" because The Canterbury Tales, though written in Middle English, is nevertheless composed in an idiom much closer to modern English than that used by other poets of his period writing in the vernacular, such as William Langland. In other words, a modern person can read and more or less understand Chaucer's English. Second, although Chaucer based his characters on stereotypical types, which was common in the Middle Ages, he created characters that became three-dimensional human beings and transcended their typing. Characters such as the Wife of Bath emerge with distinct personalities. Chaucer also was an early humorist, a strand that has been important in the English tradition and which was picked by Shakespeare, many earlier novelists such as Sterne, Fielding, and Austen and continues to be important to the present day in writers such as Ali Smith. In these various ways, he helped to move English literature forward toward modernity.