Fast I/O (Discussion)

I don't understand what's going on in here in this functions.. i have seen it may times.

    int readint()
    {
       int cc = getc(stdin);
       for (;cc < '0' || cc > '9';)  cc = getc(stdin);
       int ret = 0;
       for (;cc >= '0' && cc <= '9';)
       {
          ret = ret * 10 + cc - '0';
          cc = getc(stdin);
       }
      return ret;
   }