Thermal bridges also known as a thermal bypass are locations on a building envelope that allow hot or cold conduction to occur. They allow a path of least resistance for this temperature to penetrate the building envelope and influence the efficiency of the building.
When building energy efficient buildings we go to great lengths to ensure we manage, control or eliminate the effect of thermal bridging.
On a micro level, think about how a metal screw, which is very good at conducting cold and hot temperatures, penetrates the exterior siding and travels through the sheathing and insulation. This can have big implications in how a building envelope and siding are designed and installed.
There are a number of techniques that can be used to eliminate thermal bridging. One is the creation of a thermal break, a barrier consisting of a low conducting material that helps insulate the surrounding material.
In the photo shown you can see how GREENGirt (think of it as strapping) is used as a thermal break between the exterior siding and the interior wall. You can see the waterproof membrane and stone wool insulation adding to the rain screen and insulation system.