A century break means a player scored at least 100 points in one visit to the table. In a standard match decided by frames won, taking that frame adds one to the match score, just like winning a close tactical frame.
The first player to five wins. A 5-3 score means eight frames were played, so over 7.5 frames wins.
The winning margin is one frame. The winner would need to finish 5-3 or better for -1.5 to win.