It denotes a very LARGE number ;)
Lookup finds the largest number in the array, if all numbers are smaller than the lookup value itself. For example,
=LOOKUP(100^100,{1,2,3,4,5,50},{10,12,14,16,18,20}) will return 20, while
=LOOKUP(5,{1,2,3,4,5,50},{10,12,14,16,18,20}) will return 18.
HTH