dbExpress Tools

The DBTool Module grants you an access to SQLite advanced tools.
The  “Optimize" option , will execute the VACUUM command on the user database.
The VACUUM command rebuilds the database file, repacking it into a minimal amount of disk space.
See SQLite documentation for more information: https://sqlite.org/lang_vacuum.html

The fts5 option (Disable/enable FTS5) will let you control whether or not you want to use the SQLite full-text engine.If your hosting provider supports FTS5, then it is better to keep it enabled, but some providers are supporting SQLite without FTS5, which may have for result that your Database will simply not work. (Then disabling FTS5 will fix the issue).

Disabling FTS5 will disable the “native" full text features for your project.
For more information about FTS5, read the SQLite documentation at https://www.sqlite.org/fts5.html 

The  “Rebuild Full-Text” option will simply recalculate the FTS5 index.In general you should never have to use it, as the full text index is recalculated automatically.