Open Rates are calculated by how many people open an email and display the images. (This is the default behavior.)
Email clients don't actually send any information when an email is opened, so all email marketing services (like ConvertKit) embed a tiny 1px transparent image that is unique to every email. Once that image is loaded, the email counts as an Open.
Note: If someone has images blocked, the email will not be considered 'opened'.
Does clicking a link also count as the email being opened?
If the same person opens an email more than once, does it count as multiple Opens?
No, the tracking is unique to each email and can only register as one Open!
How is the Open Rate calculated?
The number you see as "Open Rate" in your reports is based on emails opened divided by total sent.