Acknowledgement of Country is a way of showing respect to the traditional custodians of the land on which you are standing. It is usually done at the beginning of public events, such as conferences, meetings or workshops. It is a way of acknowledging the history of the land and the traditional owners.
There is no hard and fast rule about when to do an acknowledgement of country. It is often left to the discretion of the organisers of an event. Sometimes, it is done as part of a Welcome to Country ceremony, which is performed by a traditional owner or elder.
If you are unsure whether or not to do an acknowledgement of country, it is always best to err on the side of caution and go ahead with it. It is a sign of respect and will be appreciated by the traditional owners of the land.