If you're learning French, there is a significant difference with English in that adjectives change according to masculine/feminine and singular/plural. So in your example, the gender explain the difference :
- masculine singular : "he is handsome" translate to "il est beau"
- feminine singular : "she is handsome" translate to "elle est belle"
- masculine plural : "they are handsome" translate to "ils sont beaux"
- feminine plural: "they are handsome" (with exclusively female subjects1) translate to "elles sont belles"
You may also find "bel" as a male singular form, when the adjective appears before the vowel-starting noun. For example, you will say "c'est un bel homme" and not "c'est un beau homme". If you're a starter, do not focus on this, but you may read it here and there...
Note 1: This grammatical rule known as "Le masculin l'emporte sur le féminin" raises discussion in France, as some consider it implies gender discrimination.