That means government can't endorse any particular religion, nor have an official religion. Government can't keep you from being exposed to someone else's beliefs when you're outside of your home.

It evolved out of settlers, like Quakers, who came to America to practice their faith without goverment persecution. In England, the Church of England was the official religion of the country, every citizen a subject of it. Anyone not a follower of that Church was at risk of government persecution, especially Catholics.

Many of the people settling in early American history wanted to be able to follow their faiths without government interference, even by publicly expressing it.