This metric is the overall percentage turnout for each main round of local council elections. It is calculated based on the total number of ballots reported divided by the total electorate (people eligible to vote) for the wards holding elections and is expressed as a percentage.
Local councillors are elected for 4-year terms by the local community, at the end of this term elections are held for the relevant wards. The election cycle varies for different councils, some councils elect all of their councillors at the same time (once every four years), other councils elect half or a third of their councillors at each election. Therefore this should be kept in mind when comparing the figures for different councils.