Complex sentence: Systems administrators can reduce brute force attacks by making the hashed password unreadable by unprivileged users, and the obvious way to do this is to make the passwd database itself readable only by the root user.
Split into simpler sentences: Systems administrators can reduce the likelihood of such brute force attacks by making the list of hashed passwords unreadable by unprivileged users. The obvious way to do this is to make the passwd database itself readable only by the root user.