FTP

 FTP  is an abbreviation for File Transfer Protocol, a way to move files for a website or application to a different machine (like your webserver)...

Security
FTP is the familiar term used by many people using both insecure and secure transmission technologies.

While it is true that regular FTP is insecure, most providers offering have an sFTP which is the secure version, encrypting traffic in transit and protecting credentials and wire traffic from would-be attackers.

Other security considerations are that file overwrite, both intentional and accidental become more likely.

If using sftp, having a public and private key-pair presents more data for attackers to "crack" and can present as [weak] multi-factor authentication when the password for the private key-file is set.

Deployment
FTP, or file-system snapshot / transfer as a way to get software online is very straightforward, but older method of deployment. While it does make up a part of most modern deployment patterns, it does not for example run database migrations, or automatically execute any scripts.

Particularly of concern is for websites running distributed monoliths, or  scaling out  by putting parts or copies of their service on multiple servers, FTP alone would lead to many problems.

Problems

 * Partial file transfers may break established practice around source control, leading to irreversible changes, and down-time.
 * Filesystem snapshot changes may push new files, which interact with older files, leading to hard to track-down bugs.
 * If not using a secure variant of FTP, the contents of files and in some cases plain text usernames and passwords to push to servers may be read by malicious entities seeking to use file-system access to damage websites and services.
 * I am not aware of any MFA services providing any FTP guards, although sFTP/SSH to GitHub, can use branch protection when everyone including administrators are not allowed to push to a protected branch without checks such as code review and passing automated checks.