On database host "$1", the global "sql_mode" setting does not include the "STRICT_ALL_TABLES" mode. Enabling this mode is recommended to generally improve how MySQL handles certain errors.
Without this mode enabled, MySQL will silently ignore some error conditions, including inserts which attempt to store more data in a column than actually fits. This behavior is usually undesirable and can lead to data corruption (by truncating multibyte characters in the middle), data loss (by discarding the data which does not fit into the column), or security concerns (for example, by truncating keys or credentials).
Phabricator is developed and tested in "STRICT_ALL_TABLES" mode so you should normally never encounter these situations, but may run into them if you interact with the database directly, run third-party code, develop extensions, or just encounter a bug in the software.
Enabling "STRICT_ALL_TABLES" makes MySQL raise an explicit error if one of these unusual situations does occur. This is a safer behavior and prevents these situations from causing secret, subtle, and potentially serious issues later on.
You can find more information about this mode (and how to configure it) in the MySQL manual. Usually, it is sufficient to add this to your "my.cnf" file (in the "[mysqld]" section) and then restart "mysqld":
$2 Note that if you run other applications against the same database, they may not work in strict mode.
If you can not or do not want to enable "STRICT_ALL_TABLES", you can safely ignore this warning. Phabricator will work correctly with this mode enabled or disabled.