A more thorough explanation:

Construction is the process of understanding and interpreting a law or legal document, like a contract or will. This is necessary when the language used in the document is unclear or ambiguous, or when the intent of the authors is not clear.

For example, when a court is interpreting a law, they may use a narrow construction, which means they stick closely to the literal meaning of the words used in the law. Or they may use a broad construction, which means they interpret the law more broadly, in a way that is consistent with what they believe the authors intended.

In criminal law, a court may use the rule of lenity, which means that if a criminal law is unclear or ambiguous, it should be interpreted in favor of the defendant.

For instance, in the case of Muscarello v. United States, the Supreme Court had to interpret the phrase "carries a firearm." The court could have used a narrow construction, which would have meant that the phrase only applied to people who physically carried a firearm. But instead, they used a broad construction, which meant that the phrase could also apply to people who had a firearm within reach, like in the glove compartment of a car.