When you are in the detail view of a specific email campaign you will notice two different tabs, one called Contacts (for Organization level) / Participants (for Event level) and one called Audience. The contacts tab refers to all the participants that are included in the event and related to a specific email campaign which was created. When selecting one or more contacts, you can press on the three dots to send an email. A pop up window will appear allowing you to select what email campaign to send out, whether to send it right away or schedule it, and whether to also send it to audience members or not. Make sure to click on Proceed once all the necessary information has been filled in.
Now the Audience tab is used to visualise the list of contacts that have already received or will receive emails. The list increases when emails are sent out, or scheduled to be sent out, to contacts of that specific email campaign.