What does this performance formula mean?
Just break it down into parts:


  $  \{ f(l) \mid l = 1,...,K \} $


This is a simple set construction. The above would simply create a set with all the elements from 1 to K. In your case the f(l) is the function:


  $ ||X_i-C_l||^2 $


Given the || means the norm, these are vectors you are subtracting (rows of the X and C matrices). So subtract the vectors, take the norm, and square it. That produces a new set, of which you want to take the minimum.


  $ \sum\limits_{i=1}^n $


This part is then just the sum of above min calculation for every index $i$ from $1$ to $n$.