What they mean is this:
There are connectors that plug into the 6100 chassis. The rows of pins that they connect to are called "headers" by some. Obviously they are referring to the chassis as the "board". So, this tells you they are referring to the pins on the boards. Now, these pins are mounted on the parts side, and are soldered underneath. Sometimes the solder to these pins becomes heated during use over the years and when they again cool, the (now "cold") solder will sometimes not be fully adhered to the pin. This causes a poor connection and will not always work as intended. They call this a "cold solder joint". To fix this issue, most people just put a hot soldering iron on the solder to melt it, then wait for it to "reflow" back onto the pin. What more experience techs do is completely remove the old solder from each pin and put new solder in its place - one pin at a time. It is recommended on the K6100 to do this to all the pins regardless of whether they look like they have cold solder or not...