That's the least amount of '1's and/or '0's to add to the inputstring, to achieve a multiple of five.
For example, if the input is 5, then:
5 (== '101')
Add a '0' to '101' -> '1010', equals 10 (decimal). So the shortest possible string in this case = '0'

You can also add '1111', because '101' + '1111' == '1011111' -> results in 95, but '1111' is a longer string than '0'