I must admit to being sceptical of all the hype going into this game but I have to say, for such a simple idea the Codenames delivers.
25 words in a grid, give one-word clues to your team to guess your teams words and not those of the opposition. One of the 25 words instantly causes either team to fail the game. That's about as complex as the game is. The real game is finding the right clues and how creative/abstract/tenuous you can be to make them to check off as many words as possible.
A caveat must be made, that this game is ultimately made exponentially better depending on the people you play it with; with the wrong people Codenames would have the potential to be very dry, but that is the way with most party games.