Fun in gameplay would be enjoying playing the game, not in victory or being good at it. Examples vary, but I like to play Robot Roller-Derby Disco Dodgeball; I suck at it, but it's just generally fun to play. Winning is totally unnecessary, you don't need to be good at the game. This includes games with low skill requirements, where it's fun to win despite knowing you didn't really work that hard to achieve it. If someone can start up the game for the first time, absolutely wreck everything and have fun doing it, that's still fun in gameplay.
Fun in challenge would be enjoying seeing your skills improve and in winning something you fought to win. This is present in competitive multiplayer games, but also in singleplayer games like Crypt of the Necrodancer where there's a challenge to be overcome and where definite skill improvements are obvious.
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