HuggingFace Datasets implements the ROUGE metric using the rouge_score Python package, as seen here 1.
When taking a look at the source code of this package, it seems that low, mid and high refer to confidence intervals for the scores. As seen here, there are three bounds: low (row 0), mid (row 1) and high (row 2). Mid is always the mean, while low and high bounds are specified by the confidence_interval (which defaults to 0.95 meaning it will return the 2.5th and 97.5th percentiles for a 95% confidence interval on the mean).