Introduction
Printed circuit board design is becoming increasingly complicated, as more and more functionality is required from the same space. When looking at a board, you will
often see mod wire present; especially if it is
an early revision of the board.
Purpose
Mod (modification) wire is usually very thin, often insulated with green plastic, and it will be seen crossing the board to connect two components. The reason is that the circuit board does not have the correct tracks built in to it. A piece of mod wire will often be accompanied by a cut track (where the incorrect track has been broken with a scalpel to prevent conductance).
Another common sight is where there are many, many mod wires all centered around a single chip. This is often due to the chip's being placed the wrong way round. It may be simpler to mod the board than to desolder and refit the chip.
PCBs are relatively expensive to produce, and become cheaper in large numbers - so a company will often order a hundred of their prototype board rather than 5. This way, the cost per board is lower - the tooling cost being many times higher than the actual cost per board.
If only a few modifications are required, it is much cheaper to solder a few mod wires and cut a few tracks than to completely rerun the PCB production. Later revisions have the errors fixed, and less or no mod wire.
PCB layout designers pride themselves on their Issue A boards requiring no mod wires. | 
