When in a Record View this will display a dialog box with all the fields of that record. Only one record can be selected.
When in a Summary View a temporary filter is created that filters only the records that are summarised by the selected lines of the summary view and the view is switched to a Record View. In other words it shows all the records that make up the selected summaries. Switching back to a summary view or selecting a filter (see Log - Filter) will clear this temporary filter. The easiest way is to use the Previous View option to switch back to the same Summary view that you came from.
When in a Record View this will display all the records of the visit that the selected record is part of allowing you to see exactly how the visitor navigated your site on that visit.