A matchup is an informal game term. The matchup between two decks is how they interact, usually with an eye to how one deck functions when faced with the other or which deck is favored over the other. When discussing your Movie Block Free Peoples deck and its place in the meta, you might focus on the matchup between it and common Shadow sides like Corsairs and Besiegers.
Matchups are occasionally expressed as a ratio, usually as a percentage or split out of 10. A "80/20 matchup" or "8-2" means that the speaker feels that one side is extremely heavily favored, winning 80% of the time. This usage is common in online games.