Hot & Cold Spots are issues that arise during the reflow soldering process and as the term suggests they relate to the temperature of the components or specific areas.
A Cold Spot is an area that does not get sufficient heat to activate the flux at the right time and allow the solder to reach molten stage and thus reflow.
The main reason for a Cold Spot is when Thin Chip components such as resistors are placed next to or between large/high bodied components.
This causes whats known as a shadowing effect where large components effectively shadow the smaller components from the heat in the reflow oven.
To avoid this type of issues ensure that smaller components should not placed too close to large components.
A Hot Spot is an area that gets too much heat and results in warped, blistered or even burnt boards.
Generally, this is caused by large areas of copper in specific areas.
These areas heat up quicker and become hotter than other areas of the PCB resulting in unbalanced temperatures across the PCB.
To avoid Hot Spots ensure the copper is balanced across the PCB.