What is the difference between passive FTP and active FTP?

FTP is an unusual protocol in that it uses two ports, one for commands and the other for data.
(This is one of the reasons it is superior to HTTP for transferring large files.)

Active FTP was invented first. The client initiates a connection on the server’s command port. The server then initiates a connection with the client
from its data port.

In Passive FTP, the client initiates both connections with the server, which remains “passive”.

Active FTP may cause problems if your client is behind a firewall. From the firewall’s point of view, the FTP server that is trying to initiate a connection with your client looks like an intruder and is usually blocked.
This is why many users have difficulty using FTP to download files from behind a firewall.

Passive FTP solves this problem, but creates other problems, notably where FTP server security is concerned.The server must listen on a large number of ports. This requires the firewall to let a lot of unqualified traffic through.

Most firewall administrators do not like this.
As a rule, you do not need to be concerned about passive and active FTP unless you administer an FTP server or a firewall.

If you experience difficulty using your FTP client from behind a firewall, this is likely the reason.