|| 运算符问题

CREATE OR REPLACE FUNCTION FNCDVAL
(
    xCdGrp  in varchar2,
    xCdKey  in varchar2
)
return varchar2
as
    nCdVal tb_gnr_cd.cd_val%type;
begin    select  cd_val
    into    nCdVal
    from    tb_gnr_cd
    where   tb_gnr_cd.cd_id = xCdGrp || xCdKey
    ;    return nCdVal;
end;这段代码中 “||” 的意思是什么？