Opening Your Emails
When someone receives your email and decides to click on it, that's considered opening your email. It's the first step in getting them interested in what you have say.
"Reading" or Scrolling Through Your Emails
This occurs when the recipient takes the time to go through the content of your email. It shows they're engaged and interested in your message.
Clicking on the Links Within Your Emails
If your email includes links and the recipient clicks on them, it means they're curious to learn more about what you're offering.         
Responding to Emails
When the recipient hits reply and sends a message back, it's a strong indication that they're actively interested and engaging with your content.