Secondary IP address command on the router is to actually bind another subnet to the same interface. So, lets say that you have 192.168.105.1/24 bound to an ethernet interface of a router. 192.168.106.1/24 is used else where in your network. You outgrow your subnet and need more addresses, so you bind 192.168.107.1/24 to the interface. You would essentially have 2 different subnets, bound to the same interface and the router would route both subnets. However, these subnets would be on the same broadcast domain.
This isn’t a best practice. It isn’t recommended. It is possible. I have seen a few networks where this was done as a temporary measure until the sites were re-addressed with a more contigious addressing scheme