Broadly, to diminish means to make smaller. Diminishing is the process by which something reduces in size, such as to say the "Snow diminishes with warm sunshine." The law of diminishing returns states that anything that is repeated too often, or too many times, eventually returns smaller (or diminished) rewards. An example of this could be going to an amusement park. If you go once or twice a year the experience is very exciting, filled with joy and fun. But imagine if you went to this amusement park every single day. Eventually the wonder and excitement would fade and you wouldn't experience the same levels of amusement that you initially did.