Changes the mode of a file you own, and says Operation not permitted about one you do not.

Takes an octal mode — 600, 0644, or 1777 for the sticky bit that /tmp has — or the symbolic forms: u+w, go-r, a=rx, several at once with commas. With nobody named, +w means what it means under the usual umask of 022: the owner alone, since group and other write are the bits that umask covers.

ls -l shows the result, and chmod 000 on your own note really does lock you out of reading it. Like every other change in here it is local and it is temporary: man tilenkoren-build.