Harry Potter, the hero, is shown in his ordinary world living with his uncle Vernon, aunt Petunia and their arrogant son, Dudley. Harry is clearly unhappy with his current environment.
The letter signify Harry’s initiation into Hogwarts and the wizarding world and shows the character's Call to adventure. At the same time, on Harry's 11th birthday, Hagrid visit and tells Harry that he is a wizard and that he has already been admitted into Hogwarts, the school of wizardry. This is when Harry first learns about the exsistence of magic and embarks on his journey to become a wizard.
Harry crosses the threshold when he passes over from the Muggle world into the wizarding world at Platform 9¾ crossing the barrier to the Hogwarts Express and entering into the Hogwarts School of Witchcraft and Wizardry.