and here's a man who can actually tell you what PORT and PASV is all about.
There is 2 connections involved between you and the server. The first connection is how you send the commands. The second connection is how files and directory listings are transferred. Most clients by default tell the server where to connect to get this second connection. The client sends your IP and the port that the client is listening on for the connection. However, if you're behind a firewall, the ftp server won't be able to connect.
PASV (or passive) mode is where the client asks the server where it should connect. The server will send the client the IP and port to connect to to get this data connection started.
The specific error you recieved happens when the client has NOT sent PORT OR PASV yet and you requested a directory listing or file. The server says it can't return the data cause you need to establish the data connection first using PORT or PASV.