"Leveling the playing field" is, metaphorically speaking, the removal of advantages from one or more parties in competition with one another. The metaphor concerns an unspecified sporting event, such as soccer, which would be horribly unfair if the field were sloped lengthwise. In the literal case, making a sloped field level would help ensure the outcome of the competition follows from the actual merit of the competitors relative to one another, rather than being confounded by an external factor.
Thus one usually "levels a playing field" in order to make a competition fair. However, the expression may also be used to describe an adjustment which unbalances a competition against a competitor who is known to be superior, with the intention of creating a more interesting competition. A player's handicap in golf could, for example, be called a method of "leveling the playing field" in this sense, even though, strictly speaking, it actually unbalances the game in favor of a weaker opponent.