North bridge is one of the two parts of the Intel chipset. An Intel chipset consist of a north bridge and a south bridge. The north bridge handles higher priority tasks, such as, communication between microprocessor and the memory, communicating with the PCIe slots and Level 2 cache.
The connection between the processor and the north bridge is established using frontside bus.
Since the north bridge performs such difficult tasks, it get hot very often, so a heat sink is placed on top of the chipset to avoid overheating.
This technology is not used anymore since it has been replaced with a much more sophisticated technology, which is the Intel Hub Architecture(IHA)