A smoking ceremony is a purification and cleansing custom to ward of bad spirits that uses smouldering (and typically locally sourced) native plants to produce smoke. An Aboriginal person with specialised cultural knowledge can undertake a ceremony for a variety of purposes including burials, healing and celebrations. It may take place during a Welcome to Country or Acknowledgement of Country or as part of various performances.
You can watch a Smoking Ceremony by Deadly Story which provides a great explanation into smoking ceremonies.