The concept of a board comes from a circuit board with various integrated circuits mounted on it and the usage herein generally refers to a System on a Chip (SoC) board. This concept also extends to logical (or virtual) boards for which there is no physical board.
Common boards are based on arm64 or x64 CPUs. For convenience, this directory contains arm64.gni and x64.gni which are imported into other <board>.gni files.
Further details can be found at Boards and Products