Wherever two conductors touch there is always an extra resistance, caused by the real contact area being far smaller than the apparent one and by intervening layers of oxide or dirt. In a healthy circuit breaker it sits in the tens of microhms.
It matters because the power dissipated grows with the square of the current: a few extra microhms on a busbar carrying thousands of amps translate into local heating, accelerated oxidation and, ultimately, a welded contact. Thermography catches the symptom; contact resistance measurement catches the cause.
It is measured with a micro-ohmmeter injecting high direct current, typically 100 A to 600 A, by the four-wire method. The high current is essential: only then do the surface films break down and the reading reflect real service behaviour.