This caught my attention so I looked for a source. Looks like this rate is the number of people who are Employed (60.7%) or have been actively seeking work in the last week (4%) out of the total population of people who are 16 years or older, not institutionalized, and not active military.

The source I found doesn't break down the remaining 35.3% of the population much, except to pull out about 2% as marginally attached or discouraged. Basically, people who have given up looking for work. I tried figuring out the makeup of the remaining 33%, but without much luck. How many 16 and older students are out there not working because they are full time committed to schooling? Hard to figure out, because so many students have part time jobs or are at least actively looking for part time jobs. Then, how many people are retired by choice, versus retired because there's no more work for them? How many homemakers are staying home with kids because they can't earn enough to significantly exceed childcare expenses versus those who stay home because they don't need or want to work?

Overall (all ages) labor force participation rates are low, but labor force participation among people 25-54 are very high.

All ages LFPR are probably low because the Baby Boomers are retiring and Millenials are still in college.

https://fred.stlouisfed.org/series/LNS12300060