First the the establishment clause isn't a real clause...it's an interpretation of 1A...“it would violate the Establishment Clause for the government to compel attendance or financial support of a religious institution as such, for the government to interfere with a religious organization’s selection of clergy or religious doctrine; for religious organizations or figures acting in a religious capacity to exercise governmental power”
Government should make no religious-based laws, religion should not be a part of the operating government.
Establishment Clause
You can read more here...but the clause essentially says you can't favor one religion over another in regards to policy...
