The image of human beings "trailing clouds of glory" at birth affects the meaning of this poem in the following way:
A) It suggests that, at birth, humans retain an imprint made by a divine creator.
The author states that the soul comes from somewhere else, not Earth. Unlike the English philosopher John Locke, who believed we got here as a blank slate or tabula rasa, William Wordsworth believed the soul did not get here "in entire forgetfulness" nor "in utter nakedness", but "trailing clouds of glory", that is, retaining a vivid impression of God and Heaven.
It is not until we start growing up that the "prison-house" (the mortal world) starts to make itself gradually present in our consciousness and the imprint made by our divine creator starts to die away.