Eye tracking studies are used to identify what is grabbing users’ attention. It is done using a camera that tracks eye movement. Participants of eye tracking studies complete a task or look at a website and do what they would normally do.
That data is then collected and presented in the form of heatmaps and other visualizations. You end up learning what your users are looking at, how long they are looking at it, etc. It is great because they help to display how a user perceives your product or service quantitatively. For example, if you asked your participant to forward an email, the eye tracking heat map would show you where users looked to find the forward button.
The downside of this research method is that it requires expensive equipment. For this reason, usually larger companies like Microsoft, Facebook, and Google conduct this type of study.