Open tracking lets you easily see if your emails are being opened by your customers. Open tracking adds a small 1 x 1 pixel image to the bottom of your email message which is hosted on saleslifecycle servers. When the image is downloaded we register this opening event.
Most email clients now disable images by default so open tracking is limited to those customers which are willing to 'download' these images. If your emails have other images there is a greater likelihood of the images being downloaded.
Always test your emails if you have a lot of images in an email client which hides images by default, quite often the experience for the reader is quite poor if there are too many images.