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 says that their heart "leaps up" for joy, when he sees (beholds) a rainbow in the sky. Rainbows were in the sky when he was born, now that he is a man, and he hopes - if not, he'd rather die! - that they'll be there as he grows old.
The child is like the man's father, he says, because - if you look at it chronologically - the child grows into the father, and so is in a strange way "older", belonging to the past. And, as a child is "bound" (tied) to the man he becomes, Wordsworth's speaker hopes that each day of his life will be bound to the next with "natural piety" (piousness - respectfulness - for nature [or just, "that comes naturally"]).
Hope this helps!
The poem actually means that when he sees nature, he gets excited and happy.
He hopes when he is an old man he will still enjoy the nature of life.