A problem occurs if one or more participants of a conference are behind a firewall which won't allow an IP packet containing the RTP message to pass. For this situation translators are used.
Two translators are installed, one on either side of the firewall, with the outside one funneling all multicast packets received through a secure connection to the translator inside the firewall. The translator inside the firewall sends them again as multicast packets to a multicast group restricted to the site's internal network. The following picture illustrates it -
Translator do not change SSRC or CSRC fields unlike mixers. If you don't know about SSRC and CSRC, come back to this paragraph after going through the RTP header structure.
Translators can be used for other purposes too e.g. to connect of a group of hosts speaking only IP/UDP to a group of hosts that understand only ST-II.