un chat is a masculine noun. The undefinite article 'un' or the definite article 'le' are masculine, indicating the gender of the noun. A female cat is 'une / la chatte', where the feminine articles 'une' or 'la' indicate a feminine noun.
Words ending with an additional 'e' following accented 'e' - é, ette, elle and many more are mostly categorized as feminine words, there are some exceptions. Feminine words in french: Cigarette -...
It is the same concept as in English. Such as He or She or even names... Example: if you hear the name Robert you immidiatly understand the aforesaid Robert is a boy.