The idiom ‘Greatest thing since sliced bread’ means that something is the best and most useful innovation or development invented for a long time.
Example of use: “I’ve bought a new touchscreen computer, it’s the greatest thing since sliced bread, I can’t believe I ever worked without it”.
Sliced bread was first introduced in 1928 by Otto Frederick Rohwedder from Davenport, Iowa, who invented the first loaf-at-a-time bread slicing machine. However, the first record of the idiom is thought to be in 1952, where the famous comedian Red Skelton said in an interview with the Salisbury Times: "Don't worry about television. It's the greatest thing since sliced bread".