I think that the girl on the right is very earnest and seriously minded, and she has just shared a secret that she’s a little embarrassed about (probably about a crush that she has on a boy). You can tell she’s embarrassed because she’s not making direct eye contact with her friend. The girl on the left is interested, amused, and has a hint of mockery about her (as is indicated by her upturned mouth) as such situations usually call for.
She might tease her friend a little bit for it which the other girl won’t like at all.
They are in a secluded place - the perfect place for sharing such secrets with a best friend.
This picture brings me back to when I was sharing a secret with my best friend when I was about 12. I was dreadfully earnest, and she teased me about it. It was not the response I was looking for! LOL
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“Tú, que me lees, ¿estás seguro de entender mi lenguaje?” — Jorge Luis Borges