Everytime you connect to a web page and make an HTML/CSS request your browser sends information about your computer as a way to optimize the webpage (screen size, resolution, operating system, JavaScript settings, IP, internet connection, and many more attributes). This information put together essentially forms a fingerprint that is unique to you. It can be saved and used to track you across multiple web pages without having to use cookies or other more invasive tracking methods. It’s like a digital form of facial recognition.

Here is a webpage that helps you determine how unique is your fingerprint (and therefore how identifiable):

https://amiunique.org/