Adjectives are words which MODIFY (describe) nouns
i.e. the tall girls, the vanilla cake, the green grass
In English, we rarely think of "matching" adjectives with nouns, but in Latin, this is exactly what we do.
In Latin, adjectives must AGREE with the nouns they modify in THREE ways:
CASE: if the noun is in the ablative, then the adjective is in the ablative.
NUMBER: if the noun is plural, the adjective is plural.
GENDER: if the noun is feminine, the adjective is feminine.
NOTE: Nouns and adjectives will not necessarily have the same endings.
You already know many adjective endings, because you have learned the first and second declension for nouns. Many dictionaries will list adjectives of the first and second declension in these ways:
bonus, a, um
pulcher, pulchra, pulchrum
miser, misera, miserum
You will also learn (eventually) third declension adjectives.
The good girl walks into the large town.
Bona puella in magnum oppidum ambulat.
Bona is nom sing, first declension to agree with puella
magnum is acc. sing., second declension to agree with oppidum
We see the good girl in the large town.
Bonam puellam in magno oppido spectamus.
Bonam is acc. s. first declension with puellam
magno is abl. s. second declension to agree with oppido
I walk with the good girls.
Cum bonis puellis ambulo.
Bonis is abl. pl first declension to agree with puellis