A label edge router (LER, also known as edge LSR) is a router that operates at the edge of an Multiprotocol Label Switching network.
When forwarding IP datagrams into the MPLS domain, it uses routing information to determine appropriate labels to be affixed, labels the packet accordingly, and then forwards the labelled packets into the MPLS domain.
Likewise, upon receiving a labelled packet which is destined to exit the MPLS domain, the LER strips off the label and forwards the resulting IP packet using normal IP forwarding rules.
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