The horizontal (X) axis controls the timbre of the sound. On the left, you'll often hear the transient (or tail, if reversed), while the right side gives you a more sustained version of the sound. There's also an EQ built in (darker on the left, brighter on the right).

The vertical (Y) axis controls how much the texture is affecting your sound. At the bottom, the texture is fully dry. At the top, you're hearing it fully applied.