Sweep 1/4 is a suffix call. It is only used following another call, usually connected with the word "and". The call that it follows will be one that ends with two facing couples that have some sort of circling motion. It means that they should continue with a circling motion in that same direction for another 1/4 circle (90 degrees).

For example, at the end of a Wheel and Deal from a two-faced line, the two couples have just finished "wheeling" into position to face each other. For a Wheel and Deal and Sweep 1/4 they just keep circling in the same direction, while still facing each other.

Another common case is Flutterwheel. The dancers already have a circling motion as the dancers who started on the right come back to their own side of the box. For Flutterwheel and Sweep 1/4 they just continue that circling motion for another 1/4.

Note that the action of Sweep 1/4 is similar to the four dancers doing either a Circle Left 1/4 or a Circle Right 1/4, except that they don't actually join hands to make a circle. They just stay as facing couples the whole time.