Looking for an explanation for each Bold line.

#include <stdio.h>

int a[3][4] ={

    {1,2,3,4},

    {5,6,7,8},

    {9,10,11,12}};

int b[4][3] ={

    {4,8,12},

    {3,7,11},

    {2,6,10},

    {1,5,9}};

int c[3][3];

int i,k,j,m,n,sum;

int main(){

    for(i= 0; i < 3; ++i){

        for(k = 0; k < 3; ++k){

            sum = 0;

            for (j = 0; j < 4; ++j){

                sum = sum + (a[i][j] * b[j][k]);

            }

            c[i][k] = sum;

        }

    }

    for(m = 0; m < 3; ++m){

        for (n = 0; n < 3; ++n){

            printf("%d ",c[m][n]);

        }

        printf("\n");

    }

    return 0;

}