What does it mean? What do I do about it?
If BI space in use equals the BI Threshold (in MB) the database will stop.
If you are going to use -bithold the suggested value is no more than 25% of the free space on the filesystem holding the bi file or 500MB - whichever is smaller. Crash recovery can take 2x to 3x as much bi space as was in use at the time of the crash (in order to be able to recover from a failed attempt to recover...). Repeated attempts to recover from failed crash recovery will take progressively larger amounts of space in which to succeed.