What does it mean to say the government is limited by rule of law?

That people in government should refrain from singling out particular, identifiable people for harm or benefit. The rule of law then opposes legislative privilege, or persecution. And the generality of aims, universality of application, and stability of the rule of law are intended not only to enable people to accurately predict the consequences of their actions.  Like, if you go over the speed limit, you will get a speeding ticket.  The law also states, so to promote principled governance. The rule of law is ideally an obstacle to political expediency.