What's the greatest thing since they reinvented unsliced bread?

I'm not sure about Keegan's intent, but he's referring to the old saying "That's the best thing since sliced bread!", which meant that's the best invention in a long time. He's asking what's the newest great invention.

He's also making a joke out of it by pointing out that people nowadays are discovering that you can buy bread unsliced, and that seems really cool, even though it was originally really cool that they found a way to slice bread in manufacturing.

The original saying was "The greatest thing since sliced Bread!" Now the trend is away from ready sliced and more toward the whole loaf....how bread was sold back in the day. Thus the reinventing of unsliced bread.