Let's make a graph. The horizontal axis is player skill. On the far left is no skill - just random button-pushing. On the far right is perfect videogame godhood, always doing exactly the correct thing at the correct time in the correct way. The first time you play a game, you'll probably be somewhere in the middle - farther right if you're a veteran gamer, farther left if you're a novice. As you play the game, and learn its mechanics, you'll trend right as you get better.
The vertical axis is performance level. At the very bottom is complete failure - game over as quickly as possible, not achieving any of the game's goals. Farther up is the passing line, separating failure below from success above. The line itself is a performance level of just barely passing a challenge - surviving the boss fight with one hit point left, clearing the race course just before the clock runs out, and so on. And at the very top of the axis is absolute perfect performance - winning by the largest margin possible.
Now we can chart the performance levels achievable with a particular amount of player skill: the "skill curve" for a given challenge.