James I had a Catholic mother and he had made informal promises to end the persecution (bullying) of Catholics which had started towards the end of Queen Elizabeth I's reign. Catholics worship the Catholic Church which is led by God’s representative on Earth, the Pope (who lives in the Vatican City in Rome).
When he came to throne however, King James I began to make those who did not worship the Protestant Church of England (which he was the head of) suffer even more. For example, he brought in new laws to banish Catholic priests and forced Catholics to pay large fines if they were either found to be hiding a priest in their home or didn’t attend an Anglican Church service on a Sunday.
This made many Catholics feel very disappointed and angry.
In 1604, a group of Catholics, led by Robert Catesby, decided to blow up the House of Lords at the state opening of Parliament. This would not only kill the king, but also hundreds of leading men from every county and major town in the country who would be inside too. They also planned to capture the king’s daughter Elizabeth and put her on the throne instead, as their puppet.