A pop tart is very sugary, horrible pastry. You put it in the toaster or a microwave to heat it. "Dibs!" = I get to have it! I claim it! You say it if you want something that nobody else can definitely claim. "Look there's a dollar bill on the sidewalk. Dibs!" In this case, I think the person is joking about wanting the pop tart under the refrigerator - either that, or they are VERY hungry.

A PopTart is a food. It is a toaster pastry -- a flat little pastry you stick in the toaster and then eat. Quite delicious. Dibs means that someone is claiming something. "No, but dibs" is basically like saying "I didn't know it was there, but it's mine!" or "I get the poptart!" There's a whole Wikipedia page on the concept of "dibs" if you'd like to read more.