The = (
dired-diff) command compares the current file
(the file at point) with another file (read using the minibuffer)
diff program. The file specified with the
minibuffer is the first argument of
diff, and file at point
is the second argument. The output of the
diff program is
shown in a buffer using Diff mode (see Comparing Files).
If the region is active, the default for the file read using the minibuffer is the file at the mark (i.e., the ordinary Emacs mark, not a Dired mark; see Setting the Mark). Otherwise, if the file at point has a backup file (see Backup Files), that is the default.
You could also compare files using
Major Entry Points in Ediff User’s Manual.