Grok means "to understand," of course, but Dr. Mahmoud, who might be termed the leading Terran expert on Martians, explains that it also means, "to drink" and "a hundred other English words, words which we think of as antithetical concepts. 'Grok' means all of these. It means 'fear,' it means 'love,' it means 'hate'—proper hate, for by the Martian 'map' you cannot hate anything unless you grok it, understand it so thoroughly that you merge with it and it merges with you—then you can hate it. By hating yourself. By this implies that you love it, too, and cherish it and would not have it otherwise. Then you can hate—and (I think) Martian hate is an emotion so black that the nearest human equivalent could only be called mild distaste.
Robert Heinlein's Stranger in a Strange Land introduces the term "grok" which is a bit difficult to understand - I've always viewed it as a full and total understanding of something. I'm also realizing that this is a novel I began in college but never finished and have added it to my reading queue in embarrassment....
I'm in the process of trying to grok the Fate RPG. I'm getting close - it's a system I've wrestled with for a while but I'm beginning to get it. What I've noticed is there are certain RPG systems that I just totally get. I could not play it for five years, hand me an adventure to run or a character sheet and I'd be ready to go right away (maybe give me a little time to read an adventure if you need me to run it but I'd not need time to reread the rules).
I think there's only a small number of games that meet that qualification for me. The first is of course D&D. I'd classify D&D as any version of it prior to the 3rd and any of the clones of older versions. I'm pretty close to grokking other versions of D&D, but the extreme crunchiness of 3.x tends to frustrate me when it is time to prep an adventure. That said, I'd include Dungeon Crawl Classics in my kit of grokked games.
The next game would be the D6 system. The original Star Wars RPG of course fits into this but my mini-Ghostbusters game was a blast. It was extremely easy to just pick up and play - and I'd play it again in a heartbeat.
The last of my grokked systems would be Chaosium's BRP System, especially in Call of Cthulhu. After two 7th edition campaigns I'd say that even a fairly major revision didn't break my brain. It's one of those systems that even with derivations of it like Mythras, picking it up is easy and the game is very playable.
There's also a few systems that I'm probably close to grokking. D&D 3.x and 5th edition probably fit in this category - I know them well but I definitely need a bit more time going over rules before playing them. FASA's old Star Trek and Doctor Who games are there as well - I think I played them so much in the 1980s and 1990s that even to this day I probably could run them pretty smoothly. I'm working on getting to a grokking of Fate and am thinking I'm pretty close which pleases me, given I'm really looking forward to a Dresden Files Accelerated or Atomic Robo game sometime.
Hero/Champions is probably my challenge - it's a super crunchy game that in some ways appeals to the data scientist in me - but also challenges me with the "is it worth the work" question. I might have a bit of an extended break from graduate classes in the late spring/summer time frame so it might be a good time for me to wrestle with it a bit... We'll see. I really do want to try Golden Age Champions at some point.