All traffic between peers is encrypted. You can read more about the encryption below[1]. You can also create encrypted (at rest) folders[2] for replicating via public clouds while still maintaining 100% privacy.

[1] https://help.resilio.com/hc/en-us/articles/205451025-Can-oth...

[2] https://help.resilio.com/hc/en-us/articles/207370466-Encrypt...