It's a mouthful of a word, but it basically means someone believes that the government should support a specific religion, or at the very least, that religion should have a role in government.
Here's a breakdown of the word:
* Anti: Against
* Disestablishment: The separation of church and state (as in, the removal of the establishment of a state religion)
* -arianism: A belief or doctrine
So, antidisestablishmentarianism is the belief against the separation of church and state, or the belief that a specific religion should be supported by the government.
It's important to note that this term is not commonly used in modern discourse. It's often used as a joke or a tongue twister because of its length and complexity.