In the first stanza, Wordsworth writes about how his heart leaps up when he sees a rainbow in the sky. He says that this sight makes him feel "as / A child again." This suggests that Wordsworth believes that childhood is a time of great joy and wonder, and that he hopes to recapture this feeling in adulthood.
In the second stanza, Wordsworth writes about how he loves to walk in the countryside and listen to the birds sing. He says that these experiences make him feel "calm and free," and that they help him to forget about his troubles. The speaker believes that nature can have a healing effect on the human spirit, and that it can provide us with a sense of peace and tranquility.
In the third and final stanza, Wordsworth writes about how he hopes to continue to feel this way for the rest of his life. He says that he wants to "have sight of Proteus rising from the sea; / Or hear old Triton blow his wreathed horn." These lines refer to two mythological figures who are associated with the sea. The speaker believes that if he can experience these things, he will have truly lived a full and happy life.
Overall, the poem "My Heart Leaps Up When I Behold" is about Wordsworth's hope for a longer, happier life. He believes that if he can continue to feel the joy and wonder that he felt in childhood, and if he can spend time in nature, he will be able to achieve this goal.