Forever’s documentation explains each briefly:
--minUptime Minimum uptime (millis) for a script to not be considered "spinning"
--spinSleepTime Time to wait (millis) between launches of a spinning script.
A “spinning” application is one that keeps failing/crashing shortly after a restart.
--minUptime sets the minimum amount of time that an application is expected to run. If it crashes before that limit, it’s considered to be “spinning” or problematic.
--spinSleepTime sets an amount of time that forever will wait before trying to restart a “spinning” application again.
Collectively, they help prevent having an application rapidly being restarted only to crash each time.