A registrant is someone who has completed registration for an event. Not every registrant attends, which is why organizers count registrants and attendees separately.
These are three different people who are often the same person. A registrant completed the sign-up. An attendee turned up. A ticket buyer paid, which at a corporate event is frequently an assistant who will never attend. Keeping the words separate is what makes a registration report readable.
The difference between registrants and attendees is your no-show rate, and it is the number that determines catering guarantees, room sizing and badge stock. Free events run far higher no-show rates than paid ones. Planning headcount off registrations alone is how you end up paying for two hundred lunches nobody eats.
In registration workflows, exports and reporting, where the distinction is doing real work. In marketing copy, attendee is almost always the clearer word. Switching between the two inside one page confuses readers who do not share your mental model of the database.
The Orders tab distinguishes ticket buyers from ticket holders, so you can see who paid and who is actually assigned to attend. Registrants can be added manually or imported in bulk, events that vet their audience can route sign-ups through an approval workflow before they become registrations, and registration data reporting is available separately from attendance. See ticketing and registration.