UTF-8 & Unicode, what's with 0xC0 and 0x80?

I've been reading about Unicode and UTF-8 in the last couple of days and I often come across a bitwise comparison similar to this :

int strlen_utf8(char *s) 
{
  int i = 0, j = 0;
  while (s[i]) 
  {
    if ((s[i] & 0xc0) != 0x80) j++;
    i++;
  }
  return j;
}

Can someone clarify the comparison with 0xc0 and checking if it's the most significant bit ?

Thank you!

EDIT: ANDed, not comparison, used the wrong word ;)