To "get it" or to understand something so completely that you absorb it and practically become the thing itself. In addition to getting it, "grok" also implies that one "likes it" or "appreciates it." For example, you can say, "Do you grok?" and that means, "Do you get it?" It comes from Robert Heinlein's 1961 science-fiction novel Stranger in a Strange Land and implies that one scans all available information regarding a situation, digests it, and forms a distilled opinion.
NetLingo Classification: Online Jargon