That would be the acceleration.x value. Imagine 3 lines going through your iPhone:
1) a line perpendicular to your iPhone screen and going through it, that would be Z.
2) a horizontal and parallel line to your screen, that would be X (what you re looking for).
3) a vertical and parallel line to your screen, that would be Y.
So, moving the iPhone left or right will generate changes in the X graph. Moving the iPhone up or down will generate changes in the Y graph. Moving your iPhone forward or backward will generate changes in the Z graph.
Hope this helps you grasp the accelerometer s concept,
~ Natanavra.