This was a case from way back in 1878 in which the Supreme Court ruled that Mormons could not engage in polygamy. At that time, the Mormon Church allowed polygamy, which was against the law. In his lawsuit, Reynolds argued that the government could not prevent him from practicing polygamy because his religion told him it was okay. So he was saying that having more than one wife was part of his freedom of religion.
But the Court said that freedom of religion had only to do with belief. It said you could believe whatever you want, but you could not take actions that would be illegal. The Court said that if people could do anything their religion told them, thenthat would make
the professed doctrines of religious belief superior to the law of the land, and in effect to permit every citizen to become a law unto himself. Government could exist only in name under such circumstances....
By making this ruling, the Court limited the term "free exercise of religion" to mean only that you could have freedom of belief, but not freedom of action.