Before the digital era, measurements were often made with devices like this:
A <em>clock</em> has a numbered "dial" and two pointers on it called "hands."
A <em>meter</em> also has a "dial," but the single thin pointer on it is called a "needle."
When the current (or air pressure, or whatever is being measured) changes, the needle moves up or down.
By analogy, "to move the needle" can mean "to achieve partial success," "to make gradual progress," "to improve the situation." It is especially appropriate when there is a numeric measure of success.
"Last January, our total sales in Latin America were $6 million. This year, it is still $6 million. Despite all of our hard work, we just haven't been able to move the needle."
The needle on an imaginary meter measuring sales was at the "6" mark last year, and it hasn't moved.