-s is for silent, it's to tell
cmp not to output anything¹ but only to reflect whether the files are identical or not in its exit status so that it can be used for instance in an
if shell statement:
if cmp -s file1 file2; then
echo differ or error
cmp -s file1 file2 && rm file2
If you want it to output something, remove the
cmp will output a message when the files differ stating where they start to differ.
¹ except with some implementations for error messages when files can't be opened, not when they differ.