When you select this link, you will be presented with a search box that can be used to present an alphabetic list of registrants based on starting characters you enter into the box:
By clicking the check in link, you are affirming that the registrant(s) attended the event:
If you make a mistake, just click the check out link.
If it is more convenient to just mark those who did not attend, there is a link on the right side of the screen that allows you to mass check in the whole event, assuming that you will then go through the list to check out those who did not attend:
The csv file will have a column indicating the checked in / attended status:
Checked in status can also be selected in the report builder - see Report builder.