Free-running sleep is a sleep pattern that is not adjusted (entrained) to the 24-hour cycle in nature nor to any artificial cycle.
Free-running sleep means that a person with Non-24 chooses to sleep when sleepy and to awaken spontaneously (specifically without an alarm clock or reference to the time of day). Put simply, free-running means following the body's natural rhythm.
For many with Sighted Non-24, sleeping on a normal schedule may be difficult or impossible and trying to do so results in sleep deprivation and elevated stress levels... with all the harmful health effects that accompany sleep deprivation.
Sighted Non-24 patients often find they can be much more productive and happier living on the rotating schedule to which their body naturally adheres to. For people with severe cases of Non-24, free-running may be the only realistic option.
Sources
https://en.wikipedia.org/wiki/Free-running_sleep
https://www.circadiansleepdisorders.org/docs/N24-QandA.phpI