A chipset is the primary interconnect between components not directly connected to the CPU and the CPU, it handles pretty much all data which is circled around on the secondary I/O devices. This can make the link between the chipset and the CPU a major bandwidth bottleneck. Often you only have a 4x PCIe lane wide link between it and the CPU, but about 24 lanes of PCIe devices on the chipset, though on some chipsets it is less pronounced than on others.
There was a time where a time where motherboards had a set of chips, which is why we call it chipset, where a "northbridge" handles the connection to the primary PCIe devices, the RAM and the "southbridge". The "northbridge" was connected to the CPU through the FSB (front-side bus). The "southbridge" we still have today, this component is what we call the chipset today.