Catherine of Aragon was Henry VIII's first wife. She was a cousin of the Spanish queen and therefore was a tie to the Spanish court making them allies. Catherine of Aragon was first married to Henry's older brother Arthur, though he died at a young age and the marriage was said to have not been consummated.
Henry VIII married Catherine of Aragon at his father's, Henry VII, dying wish, and as a result was happily married to her for 18 years, until he started to worry increasingly about getting a male heir to his throne as he had only one child, a daughter, Mary. Henry then invented a story about Catherine of Aragon that she in fact had consummated her marriage with Arthur therefore making their marriage wrong in the eyes of God and therefore they must divorce. The Pope didn't agree as the Spanish king held power over the papacy.
The Pope's refusal to annul the marriage was contributory to the Reformation of English churches and the eventual establishment of the Church of England, a break from the Roman Catholic Church.