The on-off spigot regulates the flow of water into your bathtub. When a government regulates an industry, they set standards the industry must follow. To regulate means to impose control.
Regulate shares a root with the word, regular. When the transit authority regulates a train schedule, it makes sure trains arrive on a more regular schedule. The pharmaceutical industry is heavily regulated –– before a drug can be marketed, it must be tested. On a more personal level, if you want to regulate your digestive system, try eating more fiber... or prunes!