What is freedom of religion?

It means the personal freedom to follow whatever religion you choose, or

NO religion if that's your choice, in any way you want as long as you don't

bother other people, without any interference from laws or other people,

and without any penalty or discrimination no matter what your choice is.

That seems like such common sense that it's hard to understand that

in huge parts of the world, that's not the way it is at all.

Freedom of religion is being able to have a religion, and express it the way YOU want and no on can discriminate you from it.