A live attendee is a person who shows up to watch a live crowdcast. If you have 60 people registered and 25 show up live during the event, then you have 25 live attendees.
Each plan includes a certain amount of live attendees already included in the price. You can have additional ones join for 15¢ each additional live attendee (per session). Events can have up to 10,000 live attendees.