Going back to square one is an idiomatic expression used in both British and American English. When you have to do this, it usually means that whatever work you’ve done on a project, product, or idea has to be tossed out and you must start fresh, or begin again. It’s usually not a fun thing to do since you likely have to discard a lot of work that hasn’t accomplished what you needed.
There are several explanations for how going back to square one became a common expression. Likely the oldest of these has to do with playing games on game boards. Certain games like "Snakes and Ladders" or "Shoots and Ladders" might make you start over from the beginning if you land on a specific space or draw a card directing you to return to the beginning. All your progress is erased, and you are on square one again, usually having to catch up to the other players in the game. This can be a particularly frustrating experience, especially for young gamers, as they see their lead or chance of winning diminish.
Another common origin story for the phrase has to do with radio broadcasts of soccer (football) matches in England during the 1920s. One of the problems with listening to a game, especially where people can assume so many different field positions, is that it can be difficult to describe to listeners which players are where at any given time.
In 1927, a man named Captain Henry Blythe Thornhill Wakelam, who later became known as Captain Teddy, became one of the first true sportscasters or commentators. In order to describe player position during soccer matches, he evolved a grid — a system of eight squares that would describe where players were. They were simply called by their number Square One, Square Eight, et cetera, and thus going back to square one could refer to being on the left side of the field near the goal, possibly a starting position after a ball changed hands. It did not mean restarting a game.
Today though, the grid reference in soccer is used less frequently, especially since most people watch rather than listen to soccer matches. The idiom remains, and is used frequently and commonly. Another idiom that can be related to this concept is the phrase “Don’t throw the baby out with the bathwater.” Sometimes when you have to start over, not everything you did has to be discarded. Scientists for instance may start with a theory that they don’t prove with certain experiments. Yet, data gathered from early experiments may not have to be thrown out, and may still be important to bear in mind if scientists must go back to square one and develop new experiments that help prove their theory.