var s: string;
begin
s:='I am looking for';
//s[11] would be "n", or same as copy(s, 11, 1)
end;
It means 11th byte (or char type) in string.
But note that string is UTF8 encoded, some 1 special character may take even 4 bytes on its own. For those cases there are UTF8Pos() and other functions.