In particular if someone gives up on looking for a job because they're too discouraged, or if someone decides to become a stay-at-home parent, this causes the size of the labor force to go down by one person.
Search for: "labor force participation rate".
Also some definitions here (US-specific): https://www.bls.gov/cps/definitions.htm
Here's also the transcript of the Planet Money podcast from March 2023 which talks about reasons folks left the workforce at that time: https://www.npr.org/transcripts/1162753771