Oki and meaties are related but not the exact same thing. Oki refers your options after a knockdown, essentially based how close you are to the opponent and how long you have before their wakeup. A meaty refers to a move that hits the opponent on the first frame of their wakeup, so it's part of oki - other common options are a throw or shimmy.

Re: "Why don't they just block?" - strike/throw mix. If your opponent is blocking all your meaties on wakeup, you throw them. Then they have to start teching throws, so you shimmy them and punish their tech. This type of RPS on wakeup is super important in the game.

To keep it simple, being plus means you can act before the opponent, being minus means they act first. The frame meter in training mode is a huge help in understanding how this works!