Most French nouns become plural by adding -s (un chat → des chats), a change that's silent in speech — plurality is usually heard in the article (des instead of un), not in the noun itself. This is a key difference from English, where the plural -s is almost always pronounced.
A handful of endings follow their own pattern: nouns ending in -eau or -eu typically add -x instead of -s (un bateau → des bateaux, un jeu → des jeux), and most nouns ending in -al change to -aux (un animal → des animaux, un journal → des journaux). Nouns already ending in -s, -x, or -z don't change at all (un fils → des fils, une voix → des voix).