GoKawiilLook at the back of a typical mountable TV and you'll find threaded mounting holes, usually arranged in a square or rectangle. The VESA pattern describes the distance between those holes, measured in millimeters.
The first number is the horizontal distance and the second is the vertical distance. So a TV with a 400 × 200 pattern has mounting holes spaced 400mm apart from left to right and 200mm apart from top to bottom. A 200 × 200 pattern measures 200mm in both directions.
The VESA numbers don't describe the TV's resolution or screen dimensions. Larger TVs often use larger mounting patterns, but screen size alone doesn't tell you which pattern a particular model uses. Checking your exact TV model is more reliable than guessing from its screen size.
You also don't need a mount limited to that exact VESA size. Many mounts support multiple VESA patterns, so you don't need a bracket labeled only for your exact size. Your TV's pattern just needs to appear on the mount's list of supported patterns.