This consists of DJB's 'Streamlined NTRU Prime' quantum-resistant
cryptosystem, currently in round 3 of the NIST post-quantum key
exchange competition; it's run in parallel with ordinary Curve25519,
and generates a shared secret combining the output of both systems.
(Hence, even if you don't trust this newfangled NTRU Prime thing at
all, it's at least no _less_ secure than the kex you were using
already.)
As the OpenSSH developers point out, key exchange is the most urgent
thing to make quantum-resistant, even before working quantum computers
big enough to break crypto become available, because a break of the
kex algorithm can be applied retroactively to recordings of your past
sessions. By contrast, authentication is a real-time protocol, and can
only be broken by a quantum computer if there's one available to
attack you _already_.
I've implemented both sides of the mechanism, so that PuTTY and Uppity
both support it. In my initial testing, the two sides can both
interoperate with the appropriate half of OpenSSH, and also (of
course, but it would be embarrassing to mess it up) with each other.
