The first time you connect to a server, a "handshake" needs to be performed in order to generate a shared secret key. If you've already performed the handshake

The first time you connect to a server, a "handshake" needs to be performed in order to generate a shared secret key. If you've already performed the handshake with a given server in the past, 0-Rtt allows you to skip it and use the key you generated before.