Prim's Algorithem, please explain

can some one explain to me what is currently happening in this loop? This is a part of a code for my assignment on Prim's algorithm.

I get the: while countIcluded is not vertices.length, part. I need to understand what's happening below. it's worth to mention that included is an array of booleans.

Expect that I'm new to this, because I am and please explain as simple as possible (if possible) so I can understand the basics.

while (countIncluded != vertices.length) {

    // find the not included vertex with minimum cost
    int u = -1;
    for (int i = 0; i < vertices.length; i++) {
        if (!included[i]) {
            if (u == -1 || C[u] > C[i])
                u = i;
        }
    }

    // include in MST
    included[u] = true;
    countIncluded++;
}

See if the comments make it cleared:

      while (countIncluded != vertices.length) { 

            //u represents a previous i value 
            int u = -1; //value serves as flag for "first time use"  

            //the purpose of this loop is to iterate over included array, which presumably 
            //if of the same length as vertices.
            for (int i = 0; i < vertices.length; i++) { 
                if (!included[i]) { //execute if the value of included[i] is false
                    /* execute if one of these apply:
                       u value is -1 : meaning that it is the first time 
                       included[i] is false (and there is not previous value to compare to).
                       OR C[u] > C[i] : previous vertex value  > current vertex value
                     */
                    if (u == -1 || C[u] > C[i])    
                                      u = i;     //keep i value, the index of the lowest vertex
                                                 //value so far 
                }
            }

            //at the end of the for loop u is the index of the lowest C[u]
            included[u] = true;
            countIncluded++;
     }