Learn French: La fille est courageuse mais paresseuse.

They are describing la fille, which is a feminine noun. In French, adjectives that end in -eux in their masculine form change to -euse in the feminine form.

What would this sentence look like if we were talking about a boy?

It would be: Le garçon est courageux mais paresseux. The adjectives change back to their masculine -eux forms.

How does the pronunciation change between the masculine and feminine forms of these adjectives?

In the masculine -eux forms (courageux, paresseux), the final x is silent. But in the feminine -euse forms (courageuse, paresseuse), you pronounce a z sound at the end.

I remember fille meaning daughter. Does it mean girl or daughter here?

It usually means girl when used with la or une (la fille, une fille). It translates to daughter when used with a possessive word like ma or sa (ma fille means my daughter). So in this sentence, it means girl.