Complex sentence: C2 and S2 are an echo of S1 and C1 respectively and after they are received the handshake is considered complete and the client and server can negotiate a connection by exchanging AMF encoded messages.
Split into simpler sentences: C2 and S2 are an echo of S1 and C1 respectively and after they are received the handshake is considered complete. At this point, the client and server can negotiate a connection by exchanging AMF encoded messages.