The safety check parameter defines the limit of records difference between the new and the last crawl as a percentage of total records (inclusive). If the new number of records is less than last number of records * (1 - maxLostRecordsPercentage / 100), the process throws a SafeReindexingError, blocking the Crawler until manual restart. You can read more about the parameter that controls this setting in this documentation.
If this is expected (due to configuration changes or content changes), then you can click "Replace production index" to update the main index (which will entirely replace the contents of your existing index).
If there are no expected major configuration/content changes that would result in this (%) reduction in records since your previous crawl, you'll need to re run the crawler to see if it continues. You'll need to click the “Cancel” and retry. Additionally, we would recommend using the URL Inspector and Monitoring tabs of your Crawler to debug.