Online shopping is a new way to shop - so someone "invented/discovered/developed" this new way to shop. Many years ago people ate bread but the bread was not sliced, so they had to pull the bread apart using their hands. Then someone invented/discovered/developed a new way to make the bread easier to handle - they cut the loaf into slices of bread. That was a small invention - not the most important invention in the history of the world, but it was a smart idea. So now we say that a new idea or invention or a new way of doing something is the best idea since "sliced bread" (the idea to slice the bread). For example: The internet is the best idea since sliced bread.

a loaf of bread cut into slices(pre-sliced) packaged and sold