Bonjour Katie !

Excellent question!
Here the answer is relatively simple: in French, adjectives agree with the noun they're referring to, even though they can sometimes be separated by 1.punctuation, 2.adverb or in such a case, by an actual 3.verb :)

1.
Fatiguée,  Aline a décidé de faire une sieste.
Tired, Aline decided to take a nap.

2. 
J'admire un paysage absolument  magnifique.
I'm admiring an absolutely magnificent landscape.

3.
Mes cousines sont  gentilles.
My cousins are kind.

Verbs whose complement refers to their subject are called verbes d'état in French.

I hope that's helpful!
Bonne journée !