This is the 5034th most frequent German word.
ann
"Ann" is not a German word.
In this case, 'an' operates as a preposition to imply the location of the action, specifically at the door.
Er klopfte an die Tür.
He knocked at the door.
Here 'an' functions as a preposition designating the spatial relation, indicating the object hangs near or against the wall.
Das Bild hängt an der Wand.
The picture hangs on the wall.
Here 'an' is used as a preposition which indicates 'about' or 'on' referring to the focus or interest in the object following it.