Definition of an Email Open
Also please note, this count is not a 100% accurate since it relies on the receipt having their images enabled/turned on in their email without that we can't record it as an open (unfortunately there is no way around that). So the total number of opens for this statistic is usually lower than what it actually is.
To recap, for emails sent through email marketing campaigns, every time one of those emails has images enabled and is opened this is recorded. The weekly total of the number of opens is displayed in our weekly stats email.
For example if I had 100 emails sent out through a campaign and 75 people opened those emails (with images enabled), then my open count would be 75.