Thanks for the question Beth. A plebiscite is a national vote on a question that does not affect the Australian Constitution. Governments can hold plebiscites to test whether people support or oppose a proposed action on an issue. The government is not bound to act on the 'result' of a plebiscite.
Only two national plebiscites have been held – in 1916 and 1917 relating to the introduction of conscription during World War I. State and territory governments have also held plebiscites on various issues.
More information about plebiscites can be found in this fact sheet.