MPLS stands for multi protocol label switching and is a way to organize and transmit your data over a wide area network.  MPLS switching is replacing frame relay as a way to connect multiple locations.

Frame relay has been a well used technology for a number of years because it was a no-brainer.  If offered a better use of resources, was faster, and cheaper to boot.  Before frame relay if you needed to connect ten sites together yo had to get ten point to point connections at each site to connect them all.  Imagine if you had more to connect.  With frame you only need one connection for each site to a frame relay router at the telco.  The router sends the data frames to the desired location based on an identifier in the header.  Now frame configurations are being phased out in favor of newer architecture.

MPLS has done the same thing even better.  A MPLS WAN labels packets as they are sent onto the network, specifying how it should be handled and routed.  This enables many traffic engineering solutions.  For instance, voip traffic, video conferencing, and even security camaras all need a low latency bandwidth speed.  Some other types of data traffic can go at a different rate without it being critical.  MPLS tags the frames with and identifier to sort out the different types of traffic and send them in the best way.

MPLS benefits over frame relay are its faster, offers more ways to engineer your traffic, and also offers Qos.