Tn3270 is a terminal emulation program for connecting to computers which use IBM 3270 terminals. Major features of the emulation include color, extended highlighting, support for different screen and font sizes, and SSL support. Some more obscure features include support for the APL character set, and support for file transfer and remote printing when tn3270 is used with an IBM VM/CMS host.
To use tn3270, the host computer must provide a telnet server which supports the extensions for 3270 terminals. On your Macintosh, you must be able to connect to the host server using TCP/IP.