An SSA is a security interest over a particular asset.
This security is perfected when it is registered on the Personal Property Security Register. The asset must be uniquely identifiable to be secured on the PPSR. Homogeneous products like stock etc can therefore not have a SSA registered against them.
It will accompany a loan agreement and allow the lender to take control of the asset which the SSA is registered over. In the case of a vehicle (for instance) this would allow the lender to repossess and sell the vehicle in the event of loan default.
A company may have several GSA's registered over it but the first GSA holder takes priority in enforcement and collection.