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In order to merge duplicates from this screen, you'll want to select two records that look to be the same person. The best way to do this is usually to search the Person's (or Organization's) name in the search bar. When you pull up the two (or more) records that have the same name, check the other tabs columns in the view to make sure that the user is really same. As we can see, not all the fields are the same across the two (or more) records (for example, the Organization name is different for these two User records, despite the fact that they represent the same person). In order to select these records for merging, you'll want to CTRL or Shift + Click on the Check Boxes next to the IDs of the records.
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