What is the Visual Basic Group Join for?

WLista = (From T In obj
          Group Join P In .PassWord
                On P.Numero Equals T.PassWord
                Into Pw = Group From P1 In Pw.DefaultIfEmpty
          Group Join A In .Atributo
                On A.Empresa Equals T.AtributoEmpresa And
                   A.Numero Equals T.AtributoNumero
                Into At = Group From A1 In At.DefaultIfEmpty