You get an uppercase
C character at the beginning of the line for a file that was created by copying another file.
For example, if you copy a file from one directory to another, the new file will have
C to show you this.
See the Emacs manual, node Operating on Files. It tells you:
Here are the file-manipulating Dired commands that operate on files.
C NEW <RET>
Copy the specified files (
dired-do-copy). The argument NEW is
the directory to copy into, or (if copying a single file) the new
name. This is like the shell command
dired-copy-preserve-time is non-
nil, then copying with this
command preserves the modification time of the old file in the
dired-recursive-copies controls whether to copy
directories recursively (like
cp -r). The default is
which means to ask before recursively copying a directory.
It is likely that you copied one or more files, creating new files. Perhaps you did this accidentally, or perhaps you thought you were creating the files in one location but you created them in another location, so you were surprised to see them there.