An email open, as the name suggests, is a record of how many times your email was opened/viewed.
How are opens tracked?
Within each email is a hidden tracking pixel, when images are downloaded in the message it is recognised as being opened.
Depending on your email client, images are downloaded differently. More often than not the recipient will have to manually download them. However, if they have whitelisted the sender, sometimes the email client downloads images automatically. Email clients that have a preview page, such as Outlook, will then load these images every time which can lead to multiple ‘opens’ from one email.
You should be aware then, that if a subscriber opens an email but doesn’t download the images, your reports will not register that open.