Specific: effect of rm -r /./*
The path you gave rm to delete was /./*. Let’s break this down:


/ The root mount point, such as Macintosh HD. This is the root of the disk, therefore an absolute path was given rather than a relative path operating on the current directory. 
./ The current directory, which is now / by the path given so far. /. is equivalent to /, so // is the path so far, except duplicate directory delimiters are combined to a single delimiter, so / so far, the same as step 1.
* Glob all files and folders at this level.


Therefore, the command given was to delete all files and folders starting at the root mount point, i.e. delete all files and folders on the entire volume.

macOS System Integrity Protection will prevent this mischief from causing much damage to the system’s core components, but your own files won’t be protected if you have given full disk access to the Terminal app. I hope you have a backup of the files you need, especially from the beginning of the disk depending when you stopped the command.