What does sent/opened mean on the campaign screen?

For each campaign, statistics are being returned regarding its performance. This data allow you to evaluate the success of the campaign and maybe fine-tune your message in order to improve your feedback. The system will share the following stats:

Sent: Sent is the number of messages that have been sent to the customer.
Opened: Opened is the number of messages which your customers have opened.
Unsubscribed: Unsubscribed is the number of customers who clicked to unsubscribe from your emails.

You should consider the following,

If 'Opened' is low, perhaps tweak your subject or time of sending your email as customers are not opening your message. 

If 'Unsubscribed' is high, you need to review your entire message as customers feel your message is unwelcome or spam.