In Lightning it is a quick action button:

In SF Classic it is a button:

Clicking on this button will open a printable, .pdf file that lists all the confirmed volunteers for the opportunity that you can use as a check-in sheet.

The check in sheet lists all the confirmed volunteers and contact information for easy reference.  It is most useful for date and time specific opportunities as it will list the volunteers coming for that specific date and time.   You can use this to have volunteers sign in and then use it as a reference when reporting attendance.