Solder is a low melting point metal used to join two metals, where the solder fills the gap and bonds to both metals. This is commonly used in electronics to bond components to the board. For a good solder joint, the solder must be brought up to the proper temperature, and the pads on the PCB (metal 1) and leads of the component (metal 2) need to be heated enough. Additionally, flux is added to the solder to remove oxides on the component leads and PCB pads to allow the solder to bond to the metal; oxides can prevent the solder from sticking.

A cold solder joint is one that does not reach the proper temperature and/or does not have enough flux, leading to the solder not bonding to the joint, having a scaly/bubbly/matte appearance, and a weaker more brittle joint. Flux also doesnt do as good a job at lower temperatures so it’s important for the joint to get hot enough, and to heat the pads/component leads too