For example in UNIX, a regular file is one that is not special in some way. Special files include symbolic links and directories. A regular file is a sequence of bytes stored permanently in a file system.
Read this answer @ UNIX & Linux stackexchange: What is a regular file?
I figure rm -i is an alias, possibly rm -i. The “regular” part doesn’t
mean anything in particular, it only means that it’s not a pipe,
device, socket or anything other “special”.it means the file is not a symlink, pipe, rand, null, cpu, etc.
Perhaps you have heard the linux philosophy everything is a text. This
isn’t literally true, but it suggests a dominant operational context
where string processing tools can be applied to filesystem elements
directly. In this case, it means that in a more literal fashion. To
see the detection step in isolation, try the command file, as in file
/etc/passwd or file /dev/null.