fuser reports the process id numbers for all processes that are accessing the /mnt directory.
The manpage on fuser gives:
c current directory.
e executable being run.
f open file. f is omitted in default display mode.
F open file for writing. F is omitted in default display
r root directory.
m mmap'ed file or shared library.
Before you close them, you might check which processes they are. The first entry [1rce] is init.
So if you kill this, you will either crash the system, or force a reboot.
NOTE: you did "fuser -m /mnt", which gives the list of pids using the filesystem the directory is in. That means it is giving the pids for those processes using root.
If you wanted to see what was using filesystems mounted in /mnt (usually /mnt/xyz) then you would want to use the command "fuser -m /mnt/*" instead.
What you did is valid... if /mnt is actually has a filesystem mounted on it. You might consider using "fuser -M /mnt" which will not list any processes if the directory is not used as a mount point. (instead it dumps a "usage" message preceded by "No process specification given")