HubSpot is able to track email opens by embedding a one-pixel invisible image into the message once it is sent. When your recipient views the email and the image loads, HubSpot counts this as an open.
Some email clients do not automatically download images, in which case the recipient must manually choose to download images for the tracking pixel to load and the open to be counted. Therefore, the open number is meant to be a general indicator of your email's success.
For HubSpot Marketing emails only, an email open will be calculated if a user clicks on a link in an email, even if images (and the tracking pixel) don't load.