Strange string encoding in C code

[dila]

Can someone explain how this encoding works, thanks

int a[]={21,19,6,17,12};
for(int i=0;i<5;++i){
  int c=0;
  for(int j=0;j<5;++j){
    c+=a[j]<<(i*j);
  }
  printf("%c",'a'+c%31);
}