Site Columns are re-usable data type model which can be used in many different content types and list definitions across multiple SharePoint sites. Site Columns shares metadata information across multiple list and libraries. If we define Site Column at root level site of site collection then it can be used across all sub sites in same site collection.
Inbuilt SharePoint site creates many site columns when we create a new site. We can find them at Site Settings -> Web Designer Galleries -> Site columns. We can create our own Site Column using Create link button from Site Columns page. Each Site Column will be mapped to a Group and Type.