Assuming you're using Midnight Commander as was guessed based on the image, MC's man page says under "Listing Mode...", that one of the available fields is this:
displays a one character wide type field. This character is similar to
what is displayed by
ls with the
-F flag -
* for executable files,
@ for links,
= for sockets,
- for character devices,
+ for block devices,
| for pipes,
~ for symbolic links to directories and
! for stale symlinks (links that point nowhere).
Underneath, it's mentioned that
For example, the Full display corresponds to this format:
half type name | size | mtime
which pretty much looks like what I get by default, so it's probably just the type indicator for an executable file.
You could test that empirically too, just create a directory with a bunch of different kinds of files and see what MC shows you.