A Session Border Controller is like a Firewall for VOIP.
They are often configured as a SIP Back-To-Back User Agent (See SIP RFC).

In addition to firewall functions they also may provide services like NAT traversal.

See: NAT and VOIP for more information.

Martyn Davies of Dialogic discusses Session Border Controllers as the often misunderstood "black magic" application for VoIP networks.