Short answer
A bridging visa is usually a temporary visa that helps a person stay lawfully in Australia while another immigration matter is being processed or resolved.
Bridging visas can cover several different situations. They may be used while a person is waiting for a decision on another visa application, while a review process is under way, while a person is arranging to leave Australia, or in some special circumstances such as human trafficking, detention, or removal-related cases.
The exact purpose depends on the type of bridging visa and the person's situation. A bridging visa does not give the same rights in every case, so the visa grant notice and conditions matter.