Supercede is a fancy-sounding word for a simple concept: if someone cuts you in line, he supercedes you. To supercede is to take something or someone's place.
To supercede is to take someone else's position. A basketball team that moves into first places supercedes the team that was in first place before. Also, non-physical things can supercede each other — one law could supercede another. Supercede is considered a variant spelling of supersede, the more common way to spell this verb.