When you are in the detailed view of a specific email campaign you will notice two different tabs, one called Contacts/Participants (depending on whether you are in Global/Event level) and one called Audience.
The Contacts/Participants tab refers to all the Contacts in your CRM / all the Participants of a specific event.
When selecting one or more contacts/participants, you can click on the Actions menu (three dots) to send the email.
A popup window will appear allowing you to select which email campaign to send out, whether to send it right away or schedule it, and tick or not a checkbox to confirm if it should also be sent to people who already received it or are scheduled to receive it (previous Audience members).
Make sure to click on Proceed once all the necessary information has been filled in.
The Audience tab is used to visualize the list of contacts/participants that have already received or are scheduled to receive the specific email. There you will find the status of the email (sent or scheduled) and see how many times the email was opened by someone.
The list increases when emails are sent or scheduled to be sent, to contacts/participants of that specific email campaign.