Chaucer is given this title for a few reasons. First, he is one of the first English poets that we know by name. During this time period, it was common not to know the author's name and label it as "anonymous," largely due to the tradition of passing stories along orally through time from one generation to the next.
Second, we give him the title because of his accomplishments. His largest and best known is The Canterbury Tales. In this work (which is not finished as he died before its completion), Chaucer creates a frame story with his poetry. The outer story is the pilgrimmage that the group is taking to pay homage to Thomas Becket at Canterbury. The inside story is a collection of short stories (the first of its kind which morphs into the short story as we know it today) focusing on the individual travelers.
Third, Chaucer, through his own life experiences, is able to give us a picture of life in his age for all levels of society. His Tales give us the knight (highest ranking of the travelers) to the plowman (revered in Chaucer's time for their importance in providing food for the public, but certainly not considered as a wealthy or sophisticated member of society). Not many authors have ever been able to do this well, and it was several hundred years before Shakespeare came along and was also able to successfully represent all members of society in a straightforward and truthful way while still being entertaining.
Geoffrey Chaucer is called the father of English literature because he was the first to write what became generally well-known and recognized poems and stories in the language of the common people of his time - medieval English. Until that time, the language of the educated and of written literature in England was Latin or French. Chaucer wrote a great many works that were well-received during his lifetime and that continue to be recognized as masterpieces, but the defining factor that sets him apart was the fact that they were written in English.
Chaucer is called the father of English poetry due to his remarkable work in literature especially in although he has written many poems one of the famous poem which was his last work "The Canterbury tale "in this poem he depicted whole society of the middle age of england ,Chaucer wrote his poem in English.