It's -5 because if you add 5 to it, you get 0:

     11111011
   + 00000101
     --------
  (1)00000000

Two's complement is just like those spinning number wheel counters, like the km counter in old cars: If you drove more than 999999 km, it would look like you drove 000000. Now, if you had a funny car where driving backwards would subtract from the km counter, starting at 000000 and driving backwards 1 km would result in 999999, therefor 999999 means -1 (you could say 000000 to 499999 is positive, 500000 to 999999 is negative).