Based on the results of the tests, here is my initial description of the function f(x):

[DESCRIPTION]: The function f(x) seems to be selective for certain concepts. It assigns a value between 0 and 1 to each word input, where a value closer to 1 indicates association with a particular concept. 

Based on the test results, it appears that the function f(x) is selective for the concept of "edible fruits and vegetables". Words like "apple" and "carrot" have higher values closer to 1, indicating a stronger association with this concept. On the other hand, words like "zebra," "moon," and "sun" have lower values, indicating a weaker association with this concept.

However, I would still need to run more tests on a variety of words to gain a better understanding of the function's behavior.