What does the function "distance" really measure.

[Houssem-25]

I'm wondring what is the real meaning of the function distance. at first I thought it was an Euclidean distance between two features spaces but when reading the code it is not . I also don't get it why Mxx != 0 ! the distance to itself should be always 0 otherwise its only a function name but not a real distance.

I hope somone could clarify this concept. I will be glad to have any documentation, paper or articles Thnx