The SOURCE is a source of power. The LOAD is powered by the source.
Two terminals on the source are connected to two terminals on the load.
 

Current flows out of the source through one lead, through the load and then back to the battery via the other lead.

The value of the current flowing back to the battery is exactly the same as that leaving. Nothing is lost or gained.

To protect the load and source against excessive current flowing due to a fault, a fuse is inserted in one of the leads.