Whereby means “by which” or “through which.” It's a formal way to explain how something happens or works.

When a teacher establishes a rule whereby students can earn extra credit by reading additional books, she's creating a system through which students can improve their grades. When a city passes a law whereby bicycles must have lights at night, the law is the method by which cyclists are required to stay visible.

You'll often see whereby in official documents, rules, or explanations of processes. A club might have a procedure whereby new members must be nominated by existing members. A science experiment might describe a method whereby plants grow faster under certain conditions.

In everyday conversation, people usually just say “where” or rephrase the sentence entirely. Instead of “We made an agreement whereby I walk the dog and you feed the cat,” most people would say “We agreed that I'd walk the dog and you'd feed the cat.” But whereby appears frequently in formal writing, legal documents, and technical explanations, so understanding it helps you grasp how systems, rules, and procedures work.