Splashing in the rain is the whole story here, and the poem is happy to keep it that simple. A girl in bright yellow rain boots jumps from puddle to puddle, and the repeated line "Splish, Splash" acts like a refrain, or the sound of her steps, coming back again and again between the stanzas.
The speaker seems to watch her with fondness, calling the rain "the natures gift" and the boots "nice." Even a "great roar," which could be thunder or the storm itself, doesn't bother her. She only listens to the sound her jumps make. The last stanza adds a small, funny detail: her boots squeak, and she laughs.
The mood is light, playful and carefree. Nothing bad happens, and the rain is something to enjoy rather than avoid. The poem's plain words and bouncing repetition feel a lot like a child's own delight in a rainy day.