It's the root of many English words, including:
* Regal: royal, fit for a king
* Regent: a person who rules in the place of a king or queen
* Regime: a system or method of government
* Regular: following a pattern or rule
* Regulation: a rule or law that controls how something is done
* Royal: relating to a king or queen
So, whenever you see "rego" in a word, think of it as relating to power, authority, and control.