Short answers, pulled from the story.
Praemunire was an English legal offence consisting of asserting or maintaining papal jurisdiction, or any other foreign jurisdiction, against the supremacy of the English monarch. It was enforced by a writ called praemunire facias, and penalties included forfeiture of lands, goods, and chattels, loss of all civil rights, and imprisonment at the royal pleasure.
The statute most commonly referred to as the Statute of Praemunire (16 Ric. 2. c. 5) was passed during the sixteenth year of the reign of Richard II, who was acquiring loans from foreign creditors and papal bulls from Rome in 1392. An earlier statute of praemunire had been enacted in 1353 during the reign of Edward III.
Henry VIII revived praemunire as a political weapon during the Protestant Reformation to break clerical allegiance to Rome. The charge was used first against individuals, then against groups of clergy, and finally against the entire English clergy as alleged agents of a foreign power. Both Lord Chancellor Cardinal Thomas Wolsey and Lord Great Chamberlain Thomas Cromwell had their downfalls precipitated by a charge of praemunire.
In classical Latin, praemunire means to fortify, safeguard, or uphold something in advance or in preference. In medieval Latin, the term was confused with praemonere, meaning to forewarn, because the writ commanded a sheriff to warn the summoned person to appear before the court. A fuller reading rooted in proper Latin is to supply support for something sooner than or instead of its proper object.
Praemunire facias ceased to be an offence in England, Wales, and Northern Ireland when the Criminal Law Act 1967 came into effect. The abolition of forfeiture as a penalty for treason and felony in 1870 had not extended to praemunire, because it was classified as a misdemeanour rather than a felony.
Boris Johnson claimed at the 2018 Conservative Party Conference that those behind the Chequers plan risked prosecution for praemunire. The Times, in a fact-check of his speech, noted that praemunire had been repealed fifty-one years before the speech was delivered.