All things being equal, parity means, basically, equality. It’s used in finance, physics, math, and even sports. When people talk about parity in a football league, for example, they mean the teams are evenly matched. Go, evenly matched team, go!
Parity comes from the same Latin root as pair, which is par, for “equal.” Parity is an equal amount of something, or an equal rank or skill level between people or teams. During the Cold War, the word parity was used to describe the equal amount of resources the US and the USSR had. So now it also means an equal amount between enemies, which is probably why sportscasters love it.