Humans are made to move seamlessly through three different planes of movement: the sagittal, frontal, and transverse (rotational). The sagittal plane consists of forward and backward movement – running, squatting, deadlifting, rowing, etc. all occur in this plane. Many programs, including most CrossFit programming, is focused on this plane.
The frontal plane is made up of any lateral movement. Exercises like a lateral lunge primarily utilize this plane and should be part of any balanced program.