Learn French: Les filles sont jeunes, mais elles sont prêtes. - The girls are young, but they are ready.

Because 'they' refers to les filles (the girls). In French, you use elles when 'they' refers to a group made entirely of females (or feminine nouns). If there were even one masculine person or thing in the group, you would use ils.

Why do we use sont after both les filles and elles?

Sont is the 'they' form of the verb être (to be). You use it with plural pronouns like elles and ils (they are), but also with plural nouns like les filles (the girls are) because plural nouns act exactly like a 'they'.

Why does jeunes have an -s at the end but no extra -e for feminine?

Adjectives have to match the noun they describe. Because les filles is plural, we add an -s. But because the base word jeune (young) already ends in an -e, it stays exactly the same for masculine and feminine.

Why is the word for ready written as prêtes instead of just prêts?

The basic masculine word for 'ready' is prêt. Because it describes les filles, you have to make it feminine by adding an -e, and plural by adding an -s. That added -e is very important for speaking: it means you pronounce the 't' sound, whereas in the masculine prêt, the 't' is silent.