The definition of SOURCE is "Any thing or place from which something comes, arises, or is obtained".
For positive battery cables the source is the positive battery terminal, where the power is obtained. However, when talking about, say your light bar wires, the source would more likely be at the fuse box or sPOD.
So for negative cables/wires the source would refer to wherever you obtain the ground from.
An electrical LOAD is an electrical component that consumes electric power from the source. Examples of loads are lights, air compressors, fridges...
It is important to remember that a fuse box can be both a source in a load. This is because it is a junction block in between your battery and electrical accessories.