Diminishing returns is the decrease in the marginal output of a production process as the amount of a single factor of production is incrementally increased, while the amounts of all other factors of production stay constant.
This fundamental law of economics plays out in most aspects of our lives. From an investing perspective, gathering more information does not help to make better decisions. And the amount of time effort and money put into acquiring this extra information eats into performance (cost of buying the information eats into the profits).
Sure, you do need to research and study before you can invest. But what to study. And what is the crucial information; the two questions you need to answer instead of trying to gather every bit of information that is out there.