The poem pictures the world as two pitchers, each one pouring into the other. It's a simple image for how people, or perhaps any two sides of life, give and receive. The speaker admits right away that the exchange isn't perfect: "alas, a little spills." There's a small, rueful shrug in that line, the acceptance that nothing passes between two things without some loss.
Then the poem turns. The "and yet" in the second stanza flips the logic we expect. Emptying yourself should leave you with less, but here the more each pitcher gives, the fuller it becomes. The speaker seems to be talking about generosity, love, or sharing: the things we hand over come back to us.
The closing "Shazam!" is playful. It works like a magician's word, treating this strange math as a bit of everyday magic. The mood is gently wry and warm, a little wistful about the spill but mostly delighted by the trick.