mlr --csv nest --explode --values --across-records -f tags tags.csv

The nest verb restructures fields; --explode splits one field's value into multiple values, and --across-records turns each value into its own row. Here a semicolon-separated tags column becomes one row per tag, the classic data-cleaning step called unnesting. The --values flag says the field holds plain values rather than key-value pairs. The inverse operation, --implode, collapses rows back into one field.