MPLS stands for Multiprotocol label switching. It is an open standard that allows businesses with a pure IP or ATM (or both) infrastructure better utilize their investments by allowing new services such as layer 2 and 3 VPN's and end-to-end QoS. Some really neat features are the layer 2 VPN's which allow you to tunnel layer 2 traffic transparently accross an IP or ATM network making them seem transparently connected (this is a great service provided by service providers to connect customers ethernet segments or VLAN's together as if the switches were directly connected). MPLS-TE or Traffic Engineering opens up many new QoS techniques to be applied to your traffic as well. MPLS is a very complex technology that cant be summed up in a single post, so I am providing some links for you that may help.
MPLS Brief Technology Overview:
http://www.cisco.com/application/pdf/en/us/guest/tech/tk428/c1482/ccmigration_09186a00800a85b9.pdf
MPLS VPN's:
http://www.cisco.com/application/pdf/en/us/guest/tech/tk891/c1482/cdccont_0900aecd80162184.pdf
MPLS QoS
http://www.cisco.com/en/US/tech/tk436/tk428/technologies_white_paper09186a00800a4455.shtml
MPLS-TE:
http://www.cisco.com/en/US/tech/tk436/tk428/technologies_white_paper09186a00800a4472.shtml
Everything else:
http://www.cisco.com/en/US/tech/tk436/tsd_technology_support_category_home.html
