The basic keywords you need to know -
Profiles - Unique records of people who will be checking in. Profiles can represent guests, students, members, customers, etc.
Lists - Stores a list of profiles (people) similar to the paper and pencil sign-in sheet. A list is typically created for an event, sign-in sheet, attendance, etc.
Check-in - A check-in record/entry for a profile. Check-ins take place on a list.
Start checking in people on an iOS device, dashboard, or share a check-in link (walkthrough videos below).