A noun names a person, place, thing or idea. Nouns are important, and we use them all the time. When you speak French, you will be using nouns constantly. The tricky thing is that every noun in French has a gender; it is either masculine or feminine. And the reason this matters is that the article, as well as any modifiers (adjectives, etc.) used with the noun, must agree with the gender of the noun; even some verbs are required to agree with the noun in gender! This means that it is essential for you to know the gender of the nouns you are using.
Most nouns referring to male animals and persons are masculine, while most nouns referring to the female animal and person are feminine. Other than this, the assignment of gender to nouns often defies logic. There is just no straightforward way of knowing whether a noun should be masculine or feminine, simply by seeing what object it names. In fact, some French nouns have a different meaning depending on the gender they have: the masculine form will have one definition, while the feminine has another.
The best way to learn noun gender is to always memorize a noun with its appropriate definite or indefinite article:
Feminine - une / la (indefinite / definite)