The race determines the races that are show in game and determines their attributes and so on. If you wanted to add a new race, this is where it would be.
Each races is defined by:
income_cap: (resource_table)
possible_research: (research_table)
race_details: race_detail_table
race_path: race_path
building_path
image_filename
projectile_path
speech_path
squad_path
taskbar_path
unit_path
race_squad_cap_table: see tables
starting_buildings: see tables
starting_squads: see tables
teamcolour_preview: table containing 15 entiites, these are the entities show in the preview drown down list in the in-game army painter.