Sweep is one of many Area Effects described in the SDE rulebook.

Sweep X attacks effect an area one additional square to the model's footprint on a side, to a distance X in front of the model.

For instance, a normal model can use a sweep 2 attack to affect the 6 squares above it in a rectangle 2 high and 3 across. Alternately, it could use the sweep 2 to effect its left, with a rectangle two wide and 3 high.

For large models, the large dimension (3 above) becomes 4, so a Sweep 2 creates a rectangle 2x4 or 4x2.