What are those "sess_*" files in tmp directory?

I am finding regulary tens of thousands temp files in the tmp folder of the webserver. For what are those files good for and how can they be deleted automatically?

Thank you!

The php session files are created and used by your web server and have nothing to do with Nextcloud itself. Ususally the files are automatically deleted after a while. Aunt G. knows much about these files: