1. Cookie is often used to store user information, but some malicious sites trying to forge a HTTP request with the correct Cookie, then it will create security problems (CSRF attacks). Here a malicious Web site for third-party cookie is a cookie behavior .
2. We are using Google Baidu website / / Facebook search, can be inserted into these search engine advertising or script code, so they can track the user.
<img src="facebook.com" style="visibility:hidden;">
The above two scenarios are third-party website for cookie application. But sometimes we do not want cookie been acquired by another site, then you can set this bar cookie SameSite property.