Elasticsearch has many settings that can cause significant performance problems if not set correctly. To prevent this from happening, Elasticsearch carries out "bootstrap checks" to ensure that these important settings have been covered. If any of the checks fail, Elasticsearch will write an error to the logs and will not start.
Here's a guide with common bootstrap checks you should know and how to configure your settings correctly to pass the checks successfully. You can read the guide here: https://opster.com/elasticsearch-glossary/elasticsearch-bootstrap-checks/