"+" in file permissions

I back up my stuff with rsync over private/public-keyed SSH, to /media/pi/backups/... to a Raspberry PI with a 1GB USB HD plugged into it as the occasional, offsite backups. When I look at the backup directories on the PI, some directories have appeared.

root@backupPi:/media/pi# ls -la /media/pi
total 20
drwxr-x---+ 5 pi   pi   4096 Apr 12  2024 .
drwxr-xr-x  3 root root 4096 Oct 21  2019 ..
drwx------  2 root root 4096 Feb 20  2023 data
drwx------  2 root root 4096 Feb 20  2023 data1
drwxr-xr-x  9 pi   pi   4096 Dec 13  2023 data2

root@backupPi:/media/pi/data1# ls -la
total 8
drwx------  2 root root 4096 Feb 20  2023 .
drwxr-x---+ 5 pi   pi   4096 Apr 12  2024 ..

... and all my stuff is in data2 (although it would have gone initially into /media/pi/data.) The + signifies ACL, according to https://unix.stackexchange.com/questions/92071 , but I can't find anything more about it. How has the + been added? How do I find out what the ACL involves?