R.O.N. is a theoretical candidate whose presence on the ballot paper gives voters the option of expressing disatisfaction with all or some of the field of candidates.  It stands for the phrase "Re-open Nominations".  In the event of R.O.N. winning a position - that is exactly what happens - a by-election is called to fill the vacant position.

for instance if a voter were only to express preferences for 4 out of 7 (real) candidates they may wish to vote in the following manner:

Candidate A       Preference 3
Candidate B       Preference 1
Candidate C
Candidate D
Candidate E       Preference 4
Candidate F
Candidate G       Preference 2
R.O.N.               Preference 5

In previous years the candidate R.O.N. is known as NOTA (none of the above)