This indicates an attempt to use the QUIC protocol.
QUIC is an experimental protocol aimed at reducing web latency over that of TCP. QUIC is very similar to TCP+TLS+SPDY implemented on UDP. It is originally designed by Google in 2012, and has then been adopted by IETF for standardization since 2016. This detects for both the original Google QUIC and the IETF QUIC.