Complex sentence: The relay agent places its own IP address in the GIADDR field of the DHCP packet, and the DHCP server sends replies to the client to the GIADDR address using unicast.
Split into simpler sentences: The relay agent stores its own IP address in the GIADDR field of the DHCP packet. When the DHCP server replies to the client, it sends the reply to the GIADDR address, again using unicast.