Meaning: It's a big deal
When a commercial bread slicer hit the market in 1928, it was a very big deal. To be perfectly clear, mankind has had the ability to cut bread for a long time. It's not as if people just grabbed whole loaves and gnawed on them like apples for 6,000 years until 1928. But up until that point, people had to slice their own bread. It became a mechanized process, with uniformly shaped and sliced loaves, and bags of sliced bread became a staple in American households in the 1930s. The phrase itself probably comes from a Wonder Bread ad campaign. The Atlantic wrote a whole story all about this one.
