Keep in mind this is not the standard, ubiquitously referenced unemployment rate issued by the BLS. That unemployment rate measures the ratio of the employed to the labor force, which famously excludes large categories of people. Some of these excluded categories, like the incarcerated population, are intuitive; others, such as those who are able to work but have given up looking for work, are mystifying.
These numbers come from a working paper by Robert Moffitt, The U.S. Employment-Population Reversal in the 2000s: Facts and Explanations. Moffitt finds that the downward trend is mostly driven by young males with low educational attainment.