In English language, nouns have either of the following genders- Masculine, Feminine and Neuter.
Nouns that refer to the male gender are said to have masculine gender. Nouns that refer to the female gender are said to have female gender. While changing the gender, the masculine is changed to feminine and the feminine is changed to masculine.
In the above given question, the noun ‘tiger’ is of masculine gender representing the male species and therefore, it is changed to ‘tigress’ which is the feminine form of ‘tiger’ representing the female species of this animal group.