If the dancers who are doing this are in any kind of line, they each end in each others spot. The dancer who starts on the left slides over into the spot where the dancer on the right started. The line as a whole doesn't move.
If the dancers are part of a circle, then the positions aren't as well-defined, especially since the circle itself is typically turning. But the two dancers are still switching relative positions.
In a line, it is always the dancers on the right who are rolling in front of the dancers on the left. In a circle, if the caller doesn't specify otherwise, it will be the girls who are rolling in front of the boys.