Question

What does the Supremacy Clause do?

Answer 1

establishes that the constitution, federal laws made pursuant to it, and treaties made under it authority, constitute the supreme law of the land.

Answer 2

Supremacy Clause, in simple words, guarantee that the Constitution, Federal Laws and Treaties made under the Constitution, are the supreme law of the country.

For example, if a State Law conflicts with, for example, a Federal Law, the Federal Law must be applied, as it's part of the supreme law of the country, and the State Law is not.