M5 Stack is a format for ESP32 tinkering by stacking modules like Arduino shields / Pi hats. I impulse bought one of those kits some time ago, there's a top module with a display and processor in it, a bottom one with a battery and keyboard adapter, 3 different keyboard modules with different layouts (alphanumeric, calculator, and Gameboy style) and I understand you can also buy other models that fit in between those like a temperature sensor or SD card or whatever, hence the "stack". Each one is a plastic rounded rectangle.