What is upstream router?

Some articles on upstream router, router:

Modem - Broadband - Bridged Mode
... internal routing use what is known as bridge mode to connect to an upstream router device ... be set to bridged mode to disable the internal built-in router in order to use an upstream router instead, such as for Multiple-WAN load balancing, Multilink PPP, or ... In bridged mode, although Ethernet cabling is used to connect the modem to the upstream router, Internet Protocol is not used for communication between the devices ...