It's basically a PC clone (well, more of a "generic" MS-DOS box, its compatibility wasn't that great) that hooks up to the Mac's serial port and uses the Mac as its monitor. (Making screen updates both slower and buggier than the real thing.) I believe it included some rudimentary functions to let you paste text between the PC's screen and the Mac clipboard and some sort of disk file transfer utility... as mentioned, Google will get you the details. The "coolest" part is probably the keyboard extender that wraps around the original 128k/512k's keyboard to add function keys and a number pad.
I think I saw one in real life once at a computer-fest thing when I was, I dunno, eleven. Other than that, well, no first hand experience.