HubSpot Sales is able to track emails by embedding a 1x1 pixel, invisible image into a message once it is sent. When the user views the email and the images load, we are able to display that open to you.
Some email clients like (Outlook or Apple Mail) have a preview pane as part of the inbox view, and if recipients scroll over your email in their inbox (even passively), this can trigger an email open notification.
Sometimes, that image will be loaded multiple times from a single email open due to how the recipient views their emails. In particular, a user could be on a server which may dynamically change that user's IP address. If the image is loaded from multiple IP addresses then a notifications may indicate that your emails were being opened by multiple people.