Active/Passive FTP describes the role of the FTP Server, i.e. if you maintain a active FTP server, the FTP Server is the active part during the whole session. When a client starts an active FTP Session, the server will initiate data transfer.
Passive FTP on the other hand, requires the client to be the active part, i.e. once control session is established it will ask for a port with which data connection will take place, and finally initiate data connection with the port received.
PS:Passive FTP is considered more secure than active FTP.
Traffic Flow Active FTP (all ports are TCP): FTP Client <-> FTP Server -------------------------- >=1024 -> 21 (Control, SYN) >=1024 <- 21 (Control) >=1024 -> 21 (Control, Use Port 3197 for example) >=1024 <- 21 (Control) 3197 <- 20 (Data, SYN) 3197 -> 20 (Data)