what does get_byte mean in postgresql

Question

create or replace function  data_to_text(data text)
returns text
AS $$
declare
    raw bytea := ('\x' || replace(data,' ',''))::bytea;
    pos integer := 0;
    len integer;
    res text := '';
begin
    while (octet_length(raw) > pos) loop
        len := (get_byte(raw, pos) -3) / 2;
        exit when len <= 0;
        if pos > 0  then
            res := res || ', ';
        end if;

        res := res || (
            select string_agg( chr(get_byte(raw, i)), '')
            from generate_series(pos+1, pos+len) i
            );
        pos := pos + len + 1;
    end loop;
    return res;
end
$$ language plpgsql;

execute it: select * from data_to_text(' 0f 30 38 46 43 43 34 00');
return 08FCC4. It's the same result as hex numbers to text.

select octet_length ('\x' || replace(' 0f 30 38 46 43 43 34 00',' ','')::bytea); return 16

Even the manual already list one example I still not don't understand get_byte.
chr is roughtly integer to character based on utf8.