Unity Mathf.PerlinNoise built-in function how it works?

My question is very simple. Im using this function to create a texture and it work good but just for curiosity i want to know how it really works. Can anyone explain how this function works and return the float values that we pass in? I noticed that when we pass for example very close float values in it sometimes the number we get back is very different and when number are very different sometimes we have close values so looks like it elaborate and give back numbers in a random way but not really because everytime if we repeat the operation with the same previous numbers the result is always the same.. Thanks a lot!