When you go back to square one, you go back to the very beginning of something.
For example, if you are writing a computer program and realize you made a big mistake in the middle that will cause everything not to work, you will have to start all over again --- you'll have to go back to square one.
Maybe you are building a fence in your yard. A terrible storm comes and knocks down the part of the fence you had already built. Now you need to start at square one again.