Third Party Cookies exist for platforms like Facebook and Google for tracking user behavior on websites outside of these properties. Third party cookies are commonly seen in advertising and analytics, with a popular usage being retargeting. For example, a 3rd Party Facebook pixel could be used by something like Amazon to track the user on Amazon but to be utilized for Facebook Ads (either for tracking or remarketing).
When looking at Google, Facebook and other advertising platforms’ Third Party Pixels the data collected from those pixels lives with the advertising platform to be utilized there.
Google and Apple are looking to eliminate 3rd Party Cookies and Targeting. Apple’s iOS 14 disables them by default.
The alternative to 3rd Party Tracking would be 1st Party Tracking.