Hi,
A open is a image view or a click. For this Acymailing ads a 1pxx1px transparent tracking image. Normally you don't embed your images, so when the image is viewed it's downloaded and Acymailing counts it as an open.
This is never 100%, since most mail clients don't show images by default, users have to tell the mail client they want to view the images. So, you have receivers that don't click and don't download your images but still read your mail and these are not counted as opens.
That being said, you can use your open rate to compare your mailings and see if changes you make have any effect. The stats give you good insights, but they are never 100% complete.