Third-party cookies are cookies on other (third-party) websites. These cookies are, for example, collected by platforms such as Facebook, LinkedIn, and Google. They do this by placing pixels. This is necessary if, for example, you want to use Google Analytics or if you want to advertise on Facebook and/or LinkedIn. By following visitors across many different websites, they learn a lot about these visitors. They build user profiles with data from many different sources.
You can see what kind of cookies a website places by going to the developer tools in your browser. Check it out!