A tigress is a female tiger. That is the plain meaning, and in writing about animals it is the only one — a nature programme will say 'the tigress and her cubs' simply to make clear which parent is being talked about. But the word is also used about women, and there you should be careful, because it means two rather different things depending on who says it. Used about a mother, it is admiring: a woman who will fight fiercely for her children is 'like a tigress'. Used about a woman in other ways, it often carries an old-fashioned idea of a woman who is fierce, dangerous or sexually powerful, and that use can sound dated or unpleasant now. English used to make many of these -ess words (lioness, actress, waitress), and most of them are fading out for people; for animals they are alive and normal, because there the sex of the animal really is the information you need.