A pre-meeting registration is where a special link is created for people to insert their details (name, email, etc.). Once registered, the host can get an email indicating a new attendant, or recieve the information with some other method. This comes in handy when you're hosting an informational meeting and you don't know who is going to show up.
Zoom has a meeting registration so you know how many people are going to show up for the call. That way you can notify them that the meeting is starting if they can't join.