It's the percentage of people aged 25-54 (prime age) who are either employed or unemployed and looking for work. It's considered a good indicator of the health of the labor market.
How insufferable and/or out of touch and/or smartass does one have to be to pretend this qualifies as popularly accessible information? I cannot even imagine...
https://www.bls.gov/news.release/empsit.t15.htm
https://fred.stlouisfed.org/series/u6rate
https://www.bls.gov/charts/employment-situation/civilian-lab...