Why do the endings to adjectives change?

Adjectives are describing words, they describe nouns (which are things, for example, a table). In French, nouns all have a gender, they are either masculine of feminine and can also be plural too. So you change the ending of your adjective according to whether the noun being described is feminine or masculine (it's gender) and to whether it's singular of plural. So when you use an adjective, if the noun is feminine and singular, you add an "e" to the end of the adjective. For example "la fille forte"(the strong girl). If it's feminine and plural you add an "es" e.g. "les filles fortes" (the strong girls). If it's masculine and singular, you don't add anything, "le garçon fort" (the strong boy). If it's masculine and plural you add an "s". "les garçons forts" (the strong boys).