When you connect to a website and need a secure connection, you make sure that connection is established in HTTPS and that it is secured with a valid certificate (represented by a lock).
FTPS is the equivalent for FTP, and it is as secure as HTTPS. It means 2 things:
the connection is encrypted in a secure channel, with SSL or TLS.
the server has a valid certificate and it's identity can be verified
How to enable FTPS with my FTP client?
Some FTP clients will try to establish a secure connection by default.
That is what Filezilla does by default:
If your FTP client does not use TLS by default or you do not want to take the risk to send your password in plaintext, then you want to turn the option "Explicit FTP over TLS".
This option will force the client to establish a secure connection, and the connection will fail otherwise.
You can also find this article by searching for:
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comment établir une connexion ftp sûre pour l'envoi des fichiers logs, connexion FTP sécurisée