One of the clauses
in the first amendment
of the Bill of Rights
. It consists of: "Congress shall make no law respecting an establishment of religion"
There is some debate over whether this prohibits the government from making a law in favor of/against certain religions, or if it just means that they may not declare an official religion. The most common interpretation seems to be the first, that the government cannot make any laws specifically about religion.
However, there are a sizeable number of people who are in favor of the second interpretation. Some of these people seem only interested in getting their religion official support, and having it be taught in public schools, along with getting prayer in schools. They often claim that the government not making any laws about religion is the equivalent of being against religion.