"Some Search results may require additional settings, like Web & App Activity or Location History".
Reading more about Web & Web app activity explains it as follows:
"If Web & App Activity is turned on, your searches and activity from other Google services are saved in your Google Account, so you may get more personalized experiences, like faster searches and more helpful app and content recommendations."[1]
[1]: https://support.google.com/websearch/answer/54068?hl=en
Edit: reworded from personalized results to personal results