The Magna Carta made the English king accountable to his subjects for the first time. It also gave the people of England the right to a criminal trial in front of their peers instead of indeterminate detention following an accusation.
The Magna Carta was introduced in 1215, when King John was facing a political crisis. Its directives made everyone in England, including the king, accountable to the law for the first time. In addition, it gave subjects a say in matters like taxation and foreign invasions. It is worth noting that most people did not benefit from this, as many were peasants working for the barons and landlords, who the Magna Carta did not include.
Initially, the Magna Carta did not have much of an impact. However, following King John's reign, Henry III introduced clauses that stated while he was able to sometimes raise taxes without his subjects' permission, other cases required their approval. This eventually led to the first parliamentary session in 1265, which is a practice that still continues in the United Kingdom.
Later influences of the Magna Carta on English government include a 14th century parliament guarantee that people had a right to a trial by a jury of their peers. In the 17th century, those who opposed King Charles used it as a basis to oppose royal authority, which had a significant impact on the monarchy's ability to rule in England.