Motherboard chipsets act as controllers for the various hardware functions a motherboard is involved with such as audio, PCI, ethernet and USB. In part they also define what hardware the motherboard will be able to use such as CPU models and memory. The chipset specifies the range or limits of what hardware the motherboard can use. There are two chips on an AMD motherboard that make up the chipset, The North Bridge chip and the South Bridge chip.
As newer, faster and more demanding hardware components are developed such as CPUs and memory the motherboard companies must develop new chipsets to recognize and cope with them properly. A "dated" chipset is one that has fallen behind in that curve, i.e. it's long in the tooth and a lot of newer hardware won't work with it or won't be fully functional.