The Civilian Unemployment Rate is the measure of how many people in a nation are unemployed.
A nation's labor force consists of the employed and the unemployed. The people that are considered employed are those who legally work for pay. The people that are considered unemployed are those who are jobless and searching for jobs. Someone who is out of work for a specific reason (they are too young, disabled, choose to quit to pursue something personal, etc.) and therefore is not actively looking for work, is not considered unemployed, as they are not apart of a nation's labor force at all.
Take a close look at the graph below.
In this graph, the unemployment rate is shown from the late 1940s to 2017. This rate is found by taking the amount of unemployed people and dividing it by the labor force, then expressing that quotient as a percentage. The labor force is then calculated by adding the amount of unemployed and employed people.
As shown in this graph, during the grey areas show a huge spike in the line, meaning that it becomes harder to find and maintain employment when the economy is suffering as it does during recessions.