Adjectives have to agree with the noun they modify (describe). If a noun is plural, it has to have a plural adjective. If a noun is feminine, it has to have a feminine adjective.
Let's try some colors to describe some items.
Le chapeau est brun. The hat is brown
Le manteau est noire. The coat is black
Le parapluie est vert. The umbrella is green.
La table est brune. The table is brown.
La valise est jaune. The suitcase is yellow.
Le col sur la robe rouge est blanc. The collar on the red dress is white.
La cravate est rouge. The tie is red.
Le veston est bleu. The jacket is blue.
La robe est blanche. The dress is white.
Le pantaloon est gris. The pants are grey.
Les gants sonts bruns. The gloves are brown.
Le mouchior est rouge. The handkerchief is red.
You will notice that the hat is masculine, and therefore "brun", while the table is feminine, and therefore "brune", and the gloves are plural, masculine, and therefore "bruns". Some adjectives do not change their form whether they are masculine or feminine. Both the tie (feminine) and the handkerchief (masculine) are "rouge".
As a rule of thumb, "e" changes a word from masculine to feminine. So, if an adjective already has an "e" at the end, it does not have to be changed.
And here are some useful adjectives to describe people. Note which adjectives change with the feminine, indicated by (e)or (euse).