Summer is treated here like a living creature with a mind of its own. It arrives quietly, "tiptoes" in as the temperature shifts, and then it turns playful and energetic. The line about "slipping, slidding, swim suit fun" is the heart of the poem, full of splashing, sliding and carefree days. The spelling "slidding" is the poem's own, and it makes the word feel a little bouncier.
Then the energy changes. Summer "gallops out of touch," moving too fast to hold on to, and it slips away as quietly as it came. The final line, "It leaves us cold again," works two ways: the weather really does turn chilly, but the speaker also seems to feel a small letdown, as if something warm and happy has been taken away. The mood is light and cheerful at first, then gently wistful. The poem suggests that the best seasons are the ones that vanish before we're ready.