It is a fork from Signal that allows more privacy such as not requiring a phone number and using an encryption key as your identifier for other people to message a decrypt your devices. The best of the lot imo but one of the least used.

Here is their litepaper summary

https://getsession.org/lightpaper

Here is their more detailed whitepaper

https://getsession.org/whitepaper

Here is some technical documentation

https://docs.oxen.io/products-built-on-oxen/session