Okay kiddo, so Lyng v. Castillo is a court case that happened because a man named Lyng wanted to do something with some land that belonged to the government, but the government said no. Lyng didn't like that, so he took the government to court, and the case ended up going all the way to the Supreme Court, which is the highest court in the United States.
The question that the Supreme Court had to answer was whether or not the government could stop Lyng from doing what he wanted to do with the land. See, Lyng wanted to cut down some trees and use the land to graze his cows, but the government said that the land was supposed to be protected because it was home to some animals that were endangered.
In the end, the Supreme Court decided that the government was allowed to stop Lyng from doing what he wanted to do with the land, even though it was his property. They said that the government had a good reason to protect the animals that lived there, and that was more important than letting Lyng do what he wanted.
So basically, Lyng v. Castillo was a court case where a man wanted to use some land a certain way, but the government said no because they wanted to protect some animals that were endangered. The Supreme Court agreed with the government and said that protecting the animals was more important than letting the man do what he wanted with his property.