My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
Or let me die!
The Child is father of the Man;
And I could wish my days to be
Bound each to each by natural piety.
The speaker in the poem is Wordsworth, thinking about being happy throughout his life.
Basically, Wordsworth is saying he becomes very happy when he sees a simple sight in nature (the "rainbow in the sky"). He was this happy as both a child ("So was is when my life began") and now as an adult ("so is it now I am a man"). Also, he wishes to remain as awestruck with nature as he grows much older ("so be it when I shall grow old").
I find that there is a use of allusion at the end of the poem, referencing Mother Nature. The text "natural piety" refers to the divinity and spirit of nature, like that of Mother Nature. Nature is the reason for Wordsworth's joy. and he hopes to continue loving it as he has since his childhood.