At least in VLC (and I assume ffmpeg is the same) the @ is used to separate multicast groups from the interface section of an address. So the following address would instruct VLC to to join the multicast group 239.255.1.2
on interface 192.168.1.10
to listen for the UDP stream:
udp://192.168.1.10@239.255.1.2:5004
See here.