I'll try to rephrase the sentence to make it easier to understand: "In terms of the U.S. Constitution, no person is above the law (although that might not be true in real life)." In other words, in theory, we Americans are all the equals under the law. However, in real, everyday life, there's a lot of inequality. Some people get treated better because they're rich, powerful, attractive, etc. Here's another example of this phrase: "Theoretically speaking, we could end poverty by taking money from the rich and giving it to the poor. However, in practice, this wouldn't work." (or, "However, realistically speaking, this wouldn't work"). I hope that helps! :)

Under the laws of the US constitution , if not in practice , no one is above the law . From the Internet > "If not in practice" > if not in reality (used to refer to what actually happens as opposed to what is meant or believed to happen).

It is traditional to contrast "theory" and "practice." In theory, light and heavy objects fall at the same rate. In practice, there will be a difference because of air resistance. The U.S. Constitution says that no person is above the law. Imagine a small city. The mayor's wife parks her car in a no-parking space. She appeals to a judge. She says "I'm the mayor's wife, I don't have to obey that law." the judge would say "Nobody is above the law." That's the theory. In practice, of course, the police would all know what the mayor's wife's car looks like and probably would not enforce the law. Your sentence means that in legal theory, no person is above the law, but in real life there can be privilege, selective enforcement, and corruption.