Trump said if he becomes president, he is going to try very hard to get rid of the rule/law/provision that if a church endorses a candidate they lose their tax exempt status. That is the tax rule that came in to effect during the President Johnson era.
In other words, a church can lose it's 501(c)(3) tax-exempt status if it engages in talk about the best candidate. Trump is going to try and get rid of it so the people can be free to speak.