The act of trying repeatedly to connect to an unavailable FTP server with little or no time between connection attempts. It can be compared to repeatedly hitting the "redial" button on a telephone when dialing a phone number that is busy until the other phone is no longer busy.
Trading servers have a limit on the number of active connections that may be present at one time. A server cannot process an unlimited number of requests, so when servers are occupied to full capacity they will deny further access until they have free space. Hammering consumes bandwidth, slowing down the server. In addition to already working at capacity, the server has to then send a busy response to the device that was unable to connect every time that device attempts a connection, which further depletes the server's resources and slows the system down in its connections with other devices.
Most FTP sites have policies against hammering and require FTP clients to set retry times at specific intervals, commonly at least 120 seconds between each attempt to connect. Most FTP sites can also monitor for devices that hammer, and once detected the server will ban access to the offending IP address either permanently or for a limited amount of time.