A "board" is a piece of hardware that offers low-level interfaces to other devices. Typically, it has GPIO pins and one or multiple SPI or I2C busses. There should be exactly one instance of a board class per hardware component in an application, but it is possible to work with multiple boards at once (i.e. when having a GPIO expander connected to the Raspberry Pi).
using Board b = Board.Create();
using GpioController controller = b.CreateGpioController();
controller.OpenPin(1);
// ...
using bus = b.CreateOrGetI2cBus(1);
// ...