What does "As goes January, so goes the year" mean?

The saying "As goes January, so goes the year" is an American stock market adage which suggests that the stock market's performance in January can be an indication of the market's performance throughout the rest of the year. This phenomenon is also called the "January Barometer". Although it is not a scientifically proven connection, some investors value it and use it as a guideline for their investment decisions.