This chip contains many functions and is the "hub" of the chipset.
It contains the Blitter, the Copper, and controls DMA traffic between the other chips and memory, specifically chipmem and slowmem. Fastmem is available only to the CPU.
It comes in various versions supporting 0.5, 1, and 2MB chipmem, but they are programmed the same way - you access f.ex. the Blitter or Copper inside the chip via the same custom chip registers.
In AGA Amigas, the corresponding chip is called Alice.