You will need a z-code interpreter to play it; on my OS X laptop I use Zoom. Just download the gostak.z5 file from that link, and open it in Zoom/your interpreter of choice.
It's a game; it's a puzzle; it's a very, very good depiction of an alien universe from the perspective of one of its inhabitants.
The reference is to the sentence "The gostak distims the doshes", which is used to illustrate how syntax can convey meaning — we don't know what a gostak is, nor what distimming is, nor what doshes are, but we do know that distimming is something a gostak does to doshes, and we know that doshes can be distimmed by a gostak. As you play the game, you uncover meaning-in-this-sense, and you learn how things are related to each other; but there is no perfect one-to-one mapping of the gostak's language to English, and I have a strong feeling that the gostak's universe is very different from ours.
I "completed" the game a few days ago, but there's still a lot to discover and speculate on, so I'm still playing it.