Wherein means “in which” or “in what.” It's a formal way to refer back to something you just mentioned and point to what's inside it or what happened during it.

You might read in a history book: “The treaty of 1783 was signed in Paris, wherein Great Britain recognized American independence.” The word wherein connects back to the treaty and tells you what was in it.

Or consider: “She described the experiment wherein she tested how plants grow in different types of soil.” Here wherein means “in which” and refers to the experiment.

This word appears most often in formal writing, legal documents, and older literature. You're unlikely to use it in everyday conversation. Instead of saying “the game wherein we scored five goals,” you'd naturally say “the game in which we scored five goals” or just “the game where we scored five goals.”

Sometimes wherein asks a question: “Wherein lies the problem?” means “In what does the problem lie?” or more simply, “What's the problem?” But again, this sounds quite formal and old-fashioned.

When you encounter wherein in your reading, you can mentally replace it with “in which,” and the sentence will make perfect sense.