The level curve designer tool allows you plot an Experience (XP) level curve and analyze it against a potential difficulty equation.
As you design your game, you will want to increase the amount of difficulty or effort required for users to progress from one level to another. Or put another way, it should be more difficult for users to move from level 100 to level 101 than it is to move from level 2 to level 3 in your game. This level designer tool will help you decide how much difficulty and time progression will be required between levels.