In Shiko Dachi (the Square Stance) both feet are moved outward a little beyond shoulder width. Both feet point outward from your center at about 30° (the feet point in opposite directions). The knees are bent significantly and your weight is caused to sink straight down so that weight is evenly distributed on both legs. Your body should be quite erect; shoulders and head should be back and the hips should be forward and over the legs (not jutting out behind you).
This stance is often used to allow you to drop your weight directly downward while maintaining correct posture and balance. You might do this to pull your opponent down to destabilize them or to initiate a throw (Nage).
The stance is only moderately effective in resisting movement in any direction. That is, the stance is neither especially good nor poor at resisting someone pushing or pulling you in any direction. For this reason, the stance should be considered a good transitional stance used to initiate some instability in your opponent, but not an effective static stance.
Shiko Dachi is very similar to the much more widely practiced Kiba Dachi. One stance could easily be considered a variant of the other.