That does not mean they are not working somehow. They just do not have a traditional job or telling government that they are looking for work.
https://study.com/academy/lesson/the-labor-force-participation-rate-equation-lesson-quiz.html
so..if you are retired, self employed, full time student or not looking for a job (wife of a rich man)...you are included in the non-participation.
My problem with that number is there is no indicator how the non-participants are categorized...so to me the number does not mean a lot.