The Act was "An Act prohibiting the proclaiming any person to be King of England or Ireland, or the Dominions thereof", saying that any person proclaiming Charles Prince of Wales (or anyone else) as King of England would be guilty of high treason. Curiously, the monarchy was not formally abolished until the following May.

What's more, the power of the monarchy had already been effectively ended. Just like all laws for the last several years, this Act had been passed by the "rump" House of Commons, without any Royal Assent or consent of any House of Lords. Further, many members of the Commons had already been excluded in "Pride's Purge" for failing to support the Army's radical position - though given the voting requirements in force, it's arguable that the Army was actually more democratic than the Commons.

Be that as it may, this Act and all other legislation during the Civil War was declared null and void after the Restoration, for failing to pass the House of Lords and secure Royal Assent.