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! :)