What is the meaning of the different fields in the output of `git ls-files -s`?

In Git, a typical line of the result returned by command git ls-files -s looks like

100755 be2c2e9b0966253096472d4b482c458bc892e493 0 .gitignore

What do those fields mean?

Look no further than the git ls-files man page:

git ls-files just outputs the filenames unless --stage is specified in which case it outputs:

       [<tag> ]<mode> <object> <stage> <file>

(The --stage flag is equivalent to -s.)

What do those fields mean?

• <mode> are the mode bits. More details in How to read the mode field of git-ls-tree's output
• <object> is the SHA of the corresponding blob, i.e. a unique identifier for the contents of the file in question.
• <stage> is the stage number, which is normally 0, but takes nonzero values for files with merge conflicts.
• <file> is simply the path to the file.

You also ask, in one of your follow-up comment,

What's the relation between the <object> and the <file>?

They're completely independent, since only the contents of a file (not its path/filename) are used to generated the hash associated with it. To convince yourself of that, you can conduct the following experiment in a toy repository:

# Set things up
$ mkdir testgit
$ cd testgit/
$ git init

# Write the same contents to two files
$ printf "foo\n" > README.md
$ printf "foo\n" > bar.txt

# Stage the two files and run git ls-files
$ git add .
$ git ls-files -s

100644 257cc5642cb1a054f08cc83f2d943e56fd3ebe99 0    README.md
100644 257cc5642cb1a054f08cc83f2d943e56fd3ebe99 0    bar.txt

Note that, even though the two files have different names, they have identical SHAs, since they have the same contents.