1.7 "On est de bons amis." Why is "de" used here? What is its meaning and function?

As title. I found this example sentence in 1.7, wondering what's the function of “de” here. Merci !

Hi Ekli,
Great question! In the sentence On est de bons amis, the word de replaces des because in French, when a plural noun is preceded by an adjective, the partitive article des changes to de. Here, amis “friends” is the plural noun, and it’s preceded by the adjective bons “good.” Without the adjective, you would say On est des amis "We are friends," but adding the adjective changes des to de, making the correct phrase On est de bons amis “We are good friends.”
This is just a little grammar rule you'll need to remember; but don't worry if you forget it, in fact, many native French speakers disregard this and would naturally just say des. You can learn more about this concept in Rocket French Level 2, lesson 8.6: https://app.rocketlanguages.com/members/products/4/lesson/5663#des-before-adjectives
I hope this helps!

Thank you very much, Jonathan! I'm glad that this will be covered in L2 8.6. I am still trying to finish up all the grammar lessons in L1.