In a Single Transferable Vote election, you can express a preference for the order that you would like your vote to be used in the election of candidates.
This means that at the counting stage, if your first preference already has sufficient support to be elected or too little support to be successful, your second preference can be taken into account, followed by your third, etc.
If at any stage you become indifferent to the remaining candidates, indicating No Further Preferences will conclude your selections.
If you are indifferent to all of the candidates running in any one of the elections, then you can choose no further preference as your first choice.
This has the effect of registering your involvement in the election, but stating that you have no preference for any of the candidates. This is known as an abstention.